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Explicit computations of Hida families via overconvergent modular symbols
- Evan P. Dummit^{1},
- Márton Hablicsek^{2},
- Robert Harron^{3}Email author,
- Lalit Jain^{4},
- Robert Pollack^{5} and
- Daniel Ross^{4}
- Received: 31 July 2016
- Accepted: 15 August 2016
- Published: 14 November 2016
Abstract
In Pollack and Stevens (Ann Sci Éc Norm Supér 44(1):1–42, 2011), efficient algorithms are given to compute with overconvergent modular symbols. These algorithms then allow for the fast computation of p-adic L-functions and have further been applied to compute rational points on elliptic curves (e.g. Darmon and Pollack in Israel J Math 153:319–354, 2006, Trifkovic in Duke Math J 135(3):415–453, 2006). In this paper, we generalize these algorithms to the case of families of overconvergent modular symbols. As a consequence, we can compute p-adic families of Hecke-eigenvalues, two-variable p-adic L-functions, L-invariants, as well as the shape and structure of ordinary Hida–Hecke algebras.
Keywords
- Overconvergent modular symbols
- Hida theory
- p-adic L-functions
- L-invariants
- Hecke algebras
Mathematical Subject Classification
- 11F33
- 11R23
- 11F67
- 11F85
1 Background
In a seminal work from the mid-80s, Hida [21, 22] introduced a theory of p-adic families of ordinary Hecke-eigenforms. This work was generalized by Coleman [12] in the mid-90s to include non-ordinary forms which ultimately led to the Coleman–Mazur [11] construction of the eigencurve—a rigid analytic space which parametrizes all finite slope Hecke-eigenforms. Over the following decade, the theory of p-adic variation of automorphic forms blossomed with multiple constructions of eigenvarieties over a wide class of reductive groups [1, 2, 4, 15, 33]. Moreover, the consequences to number theory of the existence of p-adic families of automorphic forms have been profound with the proofs of the Mazur–Tate–Teitelbaum conjecture, the Main Conjecture (for class groups or modular forms!), and the Fontaine–Mazur conjecture (just to name a few) all heavily reliant upon the theory of p-adic variation.
With that said, our current state of understanding of the shape and structure of these eigenvarieties is still quite limited. Taking the simplest example of ordinary forms on \({{\mathrm{GL}}}_2 / {\mathbb Q}\) (e.g. the setting of Hida’s original work), we do not have a good understanding of a single example of a Hecke–Hida algebra which is not simply a union of open discs.
This paper will attempt to rectify this situation at least in the case of classical Hida theory by introducing methods for computing with families of overconvergent modular symbols (which form the basis of Stevens’ construction of the eigencurve). These methods generalize the constructions of [28] where overconvergent modular symbols of a fixed weight were studied (some applications of that article appear in [14] and [31]). As a consequence, we can compute q-expansions of Hida families, two-variable p-adic L-functions, L-invariants of modular forms and their symmetric squares, and, moreover, we can get our hands on the geometry of Hida families in several non-trivial situations.
For another example, consider \(p=3\) and fix a tame level \(N=11\). There are exactly two 3-ordinary forms of weight 2 and level 33, and moreover, these forms are congruent modulo 3. In particular, the Hida family attached to these forms cannot simply be the union of two open discs (because of the congruence between the two forms). The possibilities for the geometry of this Hida family include two discs glued together at some collection of points or a double cover of weight space ramified at several points. We note this example has already appeared in several places including [10, 19, 28]. Using the methods of this paper, we were able to determine that this Hida family is a double-cover of weight space ramified at a single (non-classical) weight, and moreover, this weight is congruent to \(30060 \text { }({\text {mod}}\text { }3^{11})\).
As with overconvergent modular forms, these spaces can be p-adically interpolated over weight space. To this end, set D equal to a closed disc in weight space of radius 1/p about any tame character, and set R equal to the collection of convergent power series on D. Then \({\mathbf {D}}\hat{\otimes }R\) sits inside of the space of R-valued distributions. In particular, by evaluating at a weight k in D, we obtain a specialization map \({\mathbf {D}}\hat{\otimes }R \rightarrow {\mathbf {D}}_k\), and we refer to elements of \({\mathbf {D}}\hat{\otimes }R\) as families of distributions on D.
In this paper, we introduce methods for explicitly computing approximations to elements of \({{\mathrm{Symb}}}_{\Gamma _0}({\mathbf {D}}\hat{\otimes }R)\). In particular, we are able to compute approximations to the characteristic polynomial of any Hecke operator acting on \({{\mathrm{Symb}}}_{\Gamma _0}({\mathbf {D}}\hat{\otimes }R)^{{{\mathrm{ord}}}}\). From these computations, one can then compute q-expansions of Hida families of eigenforms. From this, one can compute L-invariants via the formulae of [13, 20, 23]. Moreover, computing two-variable p-adic L-functions is immediate once one has a family of overconvergent eigensymbols in hand as in [18]. Lastly, these computations also allow us to gain some fine control over the geometry of these Hida families in a wide variety of examples. We include several of the examples we computed below. In future work, we will implement non-trivial nebentypus (thus allowing for odd weights), coefficients in extensions of \({\mathbb Z}_p\), and computations for the prime \(p=2\). The primary bottleneck in speed is computing with p-adic polynomials in Sage. After speeding this up, our future work will also aim to compute examples more systematically.
The algorithms developed in this paper have been implemented in Sage [32] and continue to be developed on the SageMathCloud. Once sufficiently polished, the code will be submitted for inclusion into Sage.
1.1 Outline
In the following section, we introduce the relevant distribution spaces leading to the definition of the space of families of distributions, \({\mathbf {D}}\hat{\otimes }R\). In the third section, we introduce methods of working in the space \({{\mathrm{Symb}}}_{\Gamma _0}({\mathbf {D}}\hat{\otimes }R)\) including producing explicit elements in this space, forming a basis of \({{\mathrm{Symb}}}_{\Gamma _0}({\mathbf {D}}\hat{\otimes }R)^{{{\mathrm{ord}}}}\), and computing characteristic power series of Hecke operators on this ordinary subspace. In the fourth section, we explain how to carry out these computations in practice by giving a systematic method of approximating families of overconvergent modular symbols. Lastly, in the fifth section, we close with several examples which we computed via these methods.
2 Distribution modules in families
In this section, we introduce the relevant distribution spaces which will ultimately be the coefficients of our spaces of modular symbols.
2.1 Distribution spaces
We will write \({\mathbf {D}}^0\) (resp. \({\mathbf A}^{\!0}\)) for the unit ball of \({\mathbf {D}}\) (resp. \({\mathbf A}\)). Note that \(\mu \in {\mathbf {D}}^0\) if and only if \(\mu (z^j) \in {\mathbb Z}_p\) for all \(j \ge 0\).
The space \({\mathbf {D}}\) is our basic distribution space which we will ultimately study in families. But we will need to make use of some slightly fancier distribution spaces which we introduce now.
2.2 Weight space
For the remainder of the paper, let p denote an odd prime. Let \({\mathcal W}= {{\mathrm{Hom}}}({\mathbb Z}_p^\times ,{\mathbb C}_p^\times )\) denote the collection of continuous characters from \({\mathbb Z}_p^\times \) to \({\mathbb C}_p^\times \). We will refer to this as weight space. There is an injective map from \({\mathbb Z}\rightarrow {\mathcal W}\) sending k to the “raising to the kth power” character.
Since \({\mathbb Z}_p^\times \cong ({\mathbb Z}/p{\mathbb Z})^\times \times (1+p{\mathbb Z}_p)\), a character in \({\mathcal W}\) is uniquely determined by its restriction to \(({\mathbb Z}/p{\mathbb Z})^\times \) and by its value on a topological generator \(\gamma \) of \(1+p{\mathbb Z}_p\). Moreover, if \(\kappa \in {\mathcal W}\), then \(|\kappa (\gamma ) - 1| < 1\).
2.3 The weight \(\kappa \) action
Let \(\Sigma _0(p)\subseteq \text {M}_2({\mathbb Z}_p)\) denote the semigroup of matrices \(\bigl ( {\begin{matrix} a &{} b \\ c &{} d \end{matrix}} \bigr )\) of non-zero determinant with \(a \in {\mathbb Z}_p^\times \) and \(c \in p{\mathbb Z}_p\). For each \(\kappa \in {\mathcal W}\), we wish to define a “weight \(\kappa \) action” of \(\Sigma _0(p)\) on the above spaces of power series and distributions. This will allow us to eventually define Hecke actions on spaces of overconvergent modular symbols.
Now we consider the case of p-adic weights. Let \(W_m\) denote the subspace of characters \(\kappa \) in \({\mathcal W}_m\) that satisfy \(|\kappa (\gamma ) - 1| \le 1/p\) for some (and hence every) topological generator \(\gamma \) of \(1+p{\mathbb Z}_p\). Note that the classical weights—the “raising to the k-th power” characters, for \(k\in {\mathbb Z}\)—are all in \(W_m\), for some m. We can (and will) identify \(W_m\) with the closed disc of radius 1 / p around 0. For \(\kappa \in W_m\), we will define weight \(\kappa \) actions on our spaces of distributions. The key to doing this is to make sense of \(\kappa (a+cz)\) as a power series in z (see Definition 2.4 and Lemma 2.5 below).
We begin with some lemmas.
Lemma 2.1
If \({{\mathrm{ord}}}_p(x) > \frac{1}{p-1}\), then \(|\log (1+x) | = |x|\).
Proof
The condition that \({{\mathrm{ord}}}_p(x) > \frac{1}{p-1}\) forces the first term to dominate in the power series expansion of \(\log (1+x)\). \(\square \)
Lemma 2.2
Proof
Lemma 2.3
- (1)
\(F_\kappa (x)\) converges for x such that \({{\mathrm{ord}}}_p(x) > \frac{1}{p-1}\), i.e. \(F_\kappa (z) \in {\mathbf A}[r]\) for any \(r < p^{-1/(p-1)}\).
- (2)
For x with \({{\mathrm{ord}}}_p(x) > \frac{1}{p-1}\), we have \(|F_\kappa (x)| \le 1\).
- (3)For \(x \in 1+p{\mathbb Z}_p\),$$\begin{aligned} F_\kappa (x-1) = \kappa (x). \end{aligned}$$
Proof
For the second part, note that every term in the power series which defines \(F_\kappa (x)\) has valuation at least 0 for x with \(1> {{\mathrm{ord}}}_p(x) > \frac{1}{p-1}\). Thus, \(|F_\kappa (x)| \le 1\) for all x with \({{\mathrm{ord}}}_p(x)> \frac{1}{p-1}\) (by the Maximum Modulus Principle applied to any closed disc of radius between 1 / p and \(p^{-1/(p-1)}\), see [7, Proposition 3 of Section 5.1.4]).
Definition 2.4
Lemma 2.5
- (1)
\(F_{\kappa ,a,c}(x)\) converges for x such that \({{\mathrm{ord}}}_p(x) > \frac{1}{p-1} - 1\), i.e. \(F_{\kappa ,a,c}(z)\) is in \({\mathbf A}[p^h]\) for any \(h< c_p := 1 - \frac{1}{p-1}=\frac{p-2}{p-1}\),
- (2)
\(|F_{\kappa ,a,c}(x)| \le 1\) for x with \({{\mathrm{ord}}}_p(x) > \frac{1}{p-1} - 1\),
- (3)for \(x \in {\mathbb Z}_p\),$$\begin{aligned} F_{\kappa ,a,c}(x) = \kappa (a+cx). \end{aligned}$$
Proof
2.4 Power series in families over weight space
We now seek to give \({\mathbf A}\hat{\otimes }R\) the structure of a \(\Sigma _0(p)\)-module in such a way that the above map (“evaluation at \(\kappa \)”) is equivariant with respect to this action on the source and the weight \(\kappa \) action on the target. We do this by constructing a two-variable power series that interpolates \(F_{\kappa ,a,c}(z)\) as \(\kappa \) varies.
Lemma 2.6
- (1)
\(K_{a,c,m}(z,w)\) converges for z and w such that \(|z| < p^{c_p}\) and \(|w| \le 1\). That is, \(K_{a,c,m}(z,w) \in {\mathbf A}[p^h] \hat{\otimes }R\) for \(h < c_p\),
- (2)
\(|K_{a,c,m}(z,w)| \le 1\) for all such z, w,
- (3)for \(\kappa \in W_m\), we have$$\begin{aligned} \kappa (K_{a,c,m}(z,w)) = F_{\kappa ,a,c}(z). \end{aligned}$$
Proof
Lemma 2.7
Proof
This lemma follows immediately from the definition of both actions. \(\square \)
We mention here two basic properties of the automorphy factor \(K_{a,c,m}(z,w)\), both of which follow directly from the definition and which will be useful later.
Lemma 2.8
- (1)
\(K_{1,0,m}(z,w) = 1\),
- (2)
\(K_{a,c,m}(z,w)\big |_{w = 0} = \omega (a)^m\).
2.5 Distributions in families over weight space
Lemma 2.9
Proof
This formula clearly gives an action on \({\mathbf {D}}[r](R)\). To complete the proof, we must check that \({\mathbf {D}}[r] \hat{\otimes }R\subseteq {\mathbf {D}}[r](R)\) is preserved by this action. This detail is verified in [6, page 30, Remark 3.1]; we note that in [6], the notation \({\mathbf {D}}[r](R)\) refers to \({\mathbf {D}}[r] \hat{\otimes } R\). \(\square \)
Lemma 2.10
If \(\widetilde{\mu }\in {\mathcal D}^\dag (R)\), then \(\widetilde{\mu }\big |\!\left( {\begin{matrix} 1 &{} a \\ 0 &{} p \end{matrix}} \right) \in {\mathbf {D}}\hat{\otimes }R\).
Proof
2.6 Analyzing the automorphy factor
By Lemma 2.6, \(K_{a,c,m}(z,w)\) is in \({\mathbf A}\hat{\otimes }R\). In this section, we will further analyze the coefficients of this automorphy factor in order to gain better control of the \(\Sigma _0(p)\)-action on families of distributions. We begin by introducing some rings that will be useful for this purpose.
Lemma 2.11
\(S_x\) is a subring of \({\mathbb Z}_p\langle \langle x \rangle \rangle \).
Proof
Lemma 2.12
If \(f \in S_x\) with \(f(0) \in {\mathbb Z}_p^\times \), then \(f^{-1} \in S_x\).
Proof
Lemma 2.13
- (1)
f(x) converges for all x in the open disc of radius \(p^r\) centered around 0, and
- (2)
\(|f(x)| \le 1\) for all such x,
Proof
Write \(g(x) = f(x/p^r)\) which is then a power series which converges on the open unit disc of \({\mathbb C}_p\). This power series is bounded in size by 1 and thus is in \({\mathcal O}_{{\mathbb C}_p}\llbracket x\rrbracket \) (since the Gauss norm equals the sup norm). Thus, \(a_n/p^{rn} \in {\mathcal O}_{{\mathbb C}_p}\) as desired. \(\square \)
Remark 2.14
From Lemma 2.13, we have that \(S_x\) is simply the collection of \({\mathbb Q}_p\)-power series which converge on the disc of radius \(p^{c_p}\) and all of whose values have size less than or equal to 1. This gives another way to see that \(S_x\) is a ring.
Theorem 2.15
Proof
Next fix some \(w_0\) with \(|w_0| \le 1\). Then \(K_{a,c,m}(z,w_0)\) is a power series which converges on the open disc of radius \(p^{c_p}\) and all of its values have size bounded by 1 on this disc (by Lemma 2.6). Thus, by Lemma 2.13, \(R_i(w_0)\) has valuation at least \(i c_p\). But since this is true for every \(w_0\) in the closed unit disc, we have that every coefficient of \(R_i(w)\) has valuation at least \(ic_p\) (since the Gauss norm is the same as the sup norm). \(\square \)
Theorem 2.16
Proof
The following lemma will be useful later.
Lemma 2.17
Proof
3 Families of overconvergent modular symbols
3.1 Modular symbols
We review here the theory of modular symbols as formulated in [3, 18, 28]. To this end, let \(\Delta _0 := {{\mathrm{Div}}}^0({\mathbb P}^1({\mathbb Q}))\) denote the set of degree zero divisors on \({\mathbb P}^1({\mathbb Q})\) which we endow with a left action of \({{\mathrm{GL}}}_2({\mathbb Q})\) via linear fractional transformations. Let \(\Gamma \) denote a congruence subgroup of \({{\mathrm{SL}}}_2({\mathbb Z})\) and let V denote a right \(\Gamma \)-module. We define \({{\mathrm{Symb}}}_{\Gamma }(V)\), the space of V-valued modular symbols of level \(\Gamma \), to be the collection of additive homomorphisms \(\varphi : \Delta _0 \rightarrow V\) such that \(\varphi (\gamma D) = \varphi (D) \big | \gamma ^{-1}\) for all \(\gamma \in \Gamma \) and \(D \in \Delta _0\).
The space \({{\mathrm{Symb}}}_{\Gamma }({{\mathrm{Sym}}}^k({\mathbb Q}_p^2))\) is the space of classical modular symbols; the systems of Hecke-eigenvalues occurring in this space match those occurring in \(M_{k+2}(\Gamma )\) (see [5, Proposition 2.5]). The space \({{\mathrm{Symb}}}_{\Gamma _0}({\mathbf {D}}_k)\) is the space of overconvergent modular symbols; the systems of finite slope Hecke-eigenvalues occurring in this space essentially match those occurring in \(M^\dag _{k+2}(\Gamma )\), the space of overconvergent modular forms (see [29, Theorem 7.1]).
3.2 Constructing families of overconvergent modular symbols
In this section, we describe a method of producing “random” families of overconvergent modular symbols. Here we follow the methods described in [28, Section 2] to explicitly write down modular symbols.
Proposition 3.1
Proof
See [28, Corollary 2.7]. \(\square \)
Remark 3.2
In [28] explicit algorithms are given to determine the \(D_i\) and the \(\gamma _i\). This is the so-called process of “solving the Manin relations”. In the end, the \(\gamma _i\) together with the identity matrix form a subset of a full set of right coset representatives for \(\Gamma _0\) in \({{\mathrm{SL}}}_2({\mathbb Z})\), and the \(D_i\) are \({\mathbb Z}[\Gamma ]\)-generators of \(\Delta _0\).
Proposition 3.1 gives us a strategy for explicitly writing down families of overconvergent modular symbols. Just randomly pick elements \(v_1, \ldots , v_t\) in \({\mathbf {D}}\hat{\otimes }R\), and then try to solve Eq. (3.1).
We will refer to equations of the form \(w \big | \Delta = v\) as difference equations. These equations were studied in detail in [28, Section 4.2] for the the module \({\mathcal D}^\dag \). The following lemma generalizes the situation to \({\mathcal D}^\dag (R)\).
Lemma 3.3
- (1)
the map \(\Delta : {\mathcal D}^\dag (R)\rightarrow {\mathcal D}^\dag (R)\) is injective;
- (2)
if \(\widetilde{\mu }\in {{\mathrm{im}}}(\Delta )\), then \(\widetilde{\mu }({\mathbf 1}) = 0\);
- (3)
for \(\widetilde{\mu }\in {\mathcal D}^\dag (R)\) with \(\widetilde{\mu }({\mathbf 1}) = 0\), there exists a unique \(\widetilde{\nu }\in {\mathcal D}^\dag (R)\) such that \(\widetilde{\nu }\big | \Delta = \widetilde{\mu }\).
Proof
Remark 3.4
- (1)
The explicit formulas for the solution of the difference equation given in [28, Lemma 4.3] apply equally well in the case of families.
- (2)
We note in the above lemma that if \(\widetilde{\nu }\) were in the smaller space \({\mathbf {D}}\hat{\otimes }R\), there is no reason for \(\widetilde{\mu }\) to again be in the \({\mathbf {D}}\hat{\otimes }R\) as denominators naturally appear in the solution of the difference equation. These denominators are the primary reason for considering the space \({\mathcal D}^\dag (R)\) in this paper.
In our quest to write down a family of overconvergent modular symbols, we have chosen the \(v_j\) arbitrarily, and thus we still have a great deal of flexibility. The following lemma explains precisely how to choose one of the \(v_j\) more carefully to force the total measure of the right hand side of Eq. (3.1) to vanish. In the following lemma, \(\mu _j\) denotes the distribution whose j-th moment is 1 and all of whose other moments vanish.
Lemma 3.5
Proof
We begin with the case \(m \equiv 0 \text { }({\text {mod}}\text { }p-1)\). We first justify that \(v_i\) is in \({\mathcal D}^\dag (R)\). That is, if \(h = \frac{\partial }{\partial z} K_{a_i,c_i,m}(z,w) \big |_{z=0}\), we need to check that \(g/h \in R\). Note that h is simply the coefficient of z in \(K_{a_i,c_i,m}(z,w)\) and is thus in \(S_w\) by Theorem 2.16. However, h is not invertible in \(S_w\). Indeed, by Lemma 2.8 part (2), we have \(h(0) = 0\). Fortunately, g also vanishes at \(w = 0\) by the discussion immediately preceding this lemma (this is where we are using the fact that \(m \equiv 0 \text { }({\text {mod}}\text { }p-1)\)). Further, by Lemma 2.17, h / w is in \(c_i \cdot S^\times \). By Remark 3.2, we have \(c_i \ne 0\), and thus h / w is invertible in \(R\). Therefore, \(g/h = (g/w) / (h/w)\) is in \(R\).
Now onto the case of \(m \not \equiv 0 \text { }({\text {mod}}\text { }p-1)\). We again justify that \(v_i\) is in \({\mathcal D}^\dag (R)\). That is, if \(h = K_{a_i,c_i,m}(0,w)\), we need to check that \(g/(h-1) \in R\). By Theorem 2.16, \(h-1\) is in \(S_w\). Further, by Lemma 2.8 part (2), the constant term of \(h-1\) is \(\omega (a_i)^m-1\) which is a unit by assumption. Thus, by Lemma 2.12, \(h-1\) is invertible and thus \(g/(h-1) \in R\) as desired.
Remark 3.6
In the case \(m \not \equiv 0 \text { }({\text {mod}}\text { }p-1)\), if it happens that \(a_i^m \equiv 1 \text { }({\text {mod}}\text { }p)\) for every i, then a simple computation shows that \(\sum _{j=1}^t v_j \big | (\gamma _j - 1)\) has 0-th moment which vanishes in weight 0. We could thus proceed as in the case of \(m \equiv 0 \text { }({\text {mod}}\text { }p-1)\). We leave the details to the reader, but we note that in our computations we have never encountered this case. Possibly this case never occurs or only occurs in very small level.
Corollary 3.7
Proof
3.3 Ordinary families of overconvergent modular symbols
As Hida families are the primary object of interest in this paper, we now describe how to pass to the ordinary subspace of our spaces of modular symbols. To this end, recall that if X is a compact \({\mathbb Z}_p\)-module equipped with a compact operator \(U_p\), we define the ordinary subspace \(X^{{{\mathrm{ord}}}} := \bigcap _n U_p^n X\). Then \(X^{{{\mathrm{ord}}}}\) is the largest subspace of X on which \(U_p\) acts invertibly. If moreover X is profinite, then there is a canonical decomposition \(X = X^{{{\mathrm{ord}}}} \oplus X^{{{\mathrm{nil}}}}\) where \(X^{{{\mathrm{nil}}}}\) is the subspace of X on which \(U_p\) acts topologically nilpotently (see [18, Proposition 2.3]). Moreover, projection onto \(X^{{{\mathrm{ord}}}}\) is given by the operator \(e:=\lim \nolimits _{n\rightarrow \infty } U_p^{n!}\).
We note that in the ordinary case no Hecke information should be lost by working on this open disc of radius 1 / p. Indeed, as Hida families extend to all of weight space, one expects that the Hecke-eigenvalues of ordinary families of modular symbols should do the same. This fact is stated in the following theorem and proven in Appendix 6.
Theorem 3.9
- (1)\(X^{{{\mathrm{ord}}}}\) is a free \(\Lambda \)-module andfor any \(k \equiv m \text { }({\text {mod}}\text { }p-1)\),$$\begin{aligned} {{\mathrm{rank}}}_\Lambda (X^{{{\mathrm{ord}}}}) = {{\mathrm{rank}}}_{{\mathbb Z}_p}({{\mathrm{Symb}}}_{\Gamma _0}({{\mathrm{Sym}}}^k({\mathbb Z}_p^2))^{{{\mathrm{ord}}}}) \end{aligned}$$
- (2)for T any Hecke operator,has coefficients in \({\mathbb Z}_p[[W]] = {\mathbb Z}_p[[pw]]\); that is, the coefficients of this characteristic polynomial extend to the open unit disc.$$\begin{aligned} {{\mathrm{char}}}(T | X^{{{\mathrm{ord}}}}) \end{aligned}$$
3.4 Vector of total measures
In this section, we make the following simple but extremely helpful observation: an element of \(X^{{{\mathrm{ord}}}}\) is completely determined by the total measures of all of its values. Moreover, since a modular symbol is determined by its values on finitely many divisors, one only needs finitely many of these total measures to determine the symbol. We can thus express any element of \(X^{{{\mathrm{ord}}}}\) as a vector with coordinates in \(\Lambda \), and thus reduce many computations with ordinary families of overconvergent modular symbols to computations in a free module over \(\Lambda \).
Proposition 3.10
- (1)
the map \(\alpha \) is injective;
- (2)
the map \(\alpha _k\) is injective;
- (3)the induced mapis injective. Here \({\mathfrak m}\) is the maximal ideal of \(\Lambda ={\mathbb Z}_p[[w]]\).$$\begin{aligned} \overline{\alpha } : X^{{{\mathrm{ord}}}}\otimes \Lambda / {\mathfrak m}{\longrightarrow } (\Lambda /{\mathfrak m})^t \cong {\mathbb F}_p^t \end{aligned}$$
Proof
Here’s one example of the usefulness of these vector of total measure maps.
Corollary 3.11
We have that \(\left\{ \Phi _1^{{{\mathrm{ord}}}}, \dots , \Phi _j^{{{\mathrm{ord}}}} \right\} \) can be completed to a \(\Lambda \)-basis of \(X^{{{\mathrm{ord}}}}\) if and only if \(\left\{ \overline{\alpha }(\Phi _1^{{{\mathrm{ord}}}}), \dots , \overline{\alpha }(\Phi _j^{{{\mathrm{ord}}}}) \right\} \) is a linearly independent set in \({\mathbb F}_p^t\).
Proof
By Proposition 3.10, \(\overline{\alpha }(\Phi _1^{{{\mathrm{ord}}}}), \dots , \overline{\alpha }(\Phi _j^{{{\mathrm{ord}}}})\) are linearly independent in \({\mathbb F}_p^t\) if and only if the images of \(\Phi _1^{{{\mathrm{ord}}}}, \dots , \Phi _j^{{{\mathrm{ord}}}}\) in \(X^{{{\mathrm{ord}}}}/{\mathfrak m}X^{{{\mathrm{ord}}}}\) are linearly independent. By a compact version of Nakayama’s lemma, this is true if and only if \(\Phi _1^{{{\mathrm{ord}}}}, \dots , \Phi _j^{{{\mathrm{ord}}}}\) is the start of a \(\Lambda \)-basis of X. \(\square \)
3.5 Bases of \(X^{{{\mathrm{ord}}}}\) and characteristic polynomials of Hecke operators
We present here a method of computing the characteristic polynomials of Hecke operators acting on the ordinary subspace of \(X^{{{\mathrm{ord}}}}\).
We begin by describing a naive idea of how one can form a \(\Lambda \)-basis of \(X^{{{\mathrm{ord}}}}\). We first note that we can assume that we know the \(\Lambda \)-rank of \(X^{{{\mathrm{ord}}}}\) as Theorem 3.9 expresses this rank in terms of the \({\mathbb Z}_p\)-rank of some classical space of modular symbols which by standard methods is readily computed.^{3} Let us assume then that we have in hand elements \(\Phi _1^{{{\mathrm{ord}}}}, \ldots , \Phi _j^{{{\mathrm{ord}}}}\) in \(X^{{{\mathrm{ord}}}}\) which are the start of a \(\Lambda \)-basis of \(X^{{{\mathrm{ord}}}}\). We now describe how to extend this set to a full \(\Lambda \)-basis by working one element at a time.
To this end, produce some “random” element \(\Phi \) of \({{\mathrm{Symb}}}_{\Gamma _0}({\mathbf {D}}^0 \hat{\otimes }\Lambda )\)—for instance, using the methods described in Sect. 3.2. Then, as described in Sect. 3.3, by iterating \(U_p\) we can form \(\Phi ^{{{\mathrm{ord}}}}\), the projection of \(\Phi \) onto \(X^{{{\mathrm{ord}}}}\). If \(\Phi _1^{{{\mathrm{ord}}}}, \ldots , \Phi _j^{{{\mathrm{ord}}}}\) together with \(\Phi ^{{{\mathrm{ord}}}}\) still form the beginning of some \(\Lambda \)-basis of \(X^{{{\mathrm{ord}}}}\) (which we can test via Corollary 3.11), we have succeeded in extending our partial basis. Otherwise, we produce another “random” symbol \(\Phi \) and continue repeating this process. As long as our method of producing such symbols is sufficiently random, we will eventually find a symbol \(\Phi ^{{{\mathrm{ord}}}}\) which extends our partial basis.
We note that even though the matrix associated to T will be defined over \(\Lambda \), by Theorem 3.9, the characteristic polynomials of these matrices will lie in \({\mathbb Z}_p[[pw]]\), and thus extend to all of weight space.
Lastly we mention that the above method works equally well for the plus and minus subspace \((X^{{{\mathrm{ord}}}})^\pm \) by simply passing to the ±-parts of the random symbols produced.
3.6 Restricting to collections of congruent forms
The methods of the previous section describe how to form a basis of \(X^{{{\mathrm{ord}}}}\) and how to compute the Hecke action on this basis. In the special case when the dimension of \((X^{{{\mathrm{ord}}}})^\pm \) is 1, our single basis element is then an eigensymbol, and thus immediately contains the information of families of Hecke eigenvalues. However, it is extremely rare for \((X^{{{\mathrm{ord}}}})^\pm \) to be one-dimensional; this only happens for small primes and small tame level. To partially circumvent this problem, we now describe a decomposition of \(X^{{{\mathrm{ord}}}}\) into Hecke stable subspaces, comprising of congruent families, and it is not at all uncommon for pieces of this decomposition to be 1-dimensional.
Let \({\mathbb T}\) denote the Hecke algebra over \(\Lambda \) acting on \(X^{{{\mathrm{ord}}}}\). The ring \({\mathbb T}\) is a semi-local ring with \({\mathbb T}\simeq \oplus _{\mathfrak m}{\mathbb T}_{{\mathfrak m}}\) where \({\mathfrak m}\) varies over the maximal ideals of \({\mathbb T}\). This isomorphism induces a Hecke-equivariant isomorphism \(X^{{{\mathrm{ord}}}}\simeq \oplus _{{\mathfrak m}} X^{{{\mathrm{ord}}}}_{{\mathfrak m}}\). We now describe how to compute the characteristic polynomials of Hecke operators acting on \(X^{{{\mathrm{ord}}}}_{{\mathfrak m}}\) for each individual maximal ideal \({\mathfrak m}\).
Fix a prime \(\ell \) and let T denote either \(T_\ell \) or \(U_\ell \) depending on whether or not \(\ell \) divides Np. Set \(\overline{f}_{{\mathfrak m},\ell }\) equal to the characteristic polynomial of T acting on \(X^{{{\mathrm{ord}}}}/{\mathfrak m}X^{{{\mathrm{ord}}}}\) which is a polynomial defined over \({\mathbb F}_p\).^{4} For a fixed \({\mathfrak m}\), one can find a prime \(\ell \) so that any lift \({f}_{{\mathfrak m},\ell }(T)\) of \(\overline{f}_{{\mathfrak m},\ell }(T)\) to characteristic 0 acts topologically nilpotently on \(X^{{{\mathrm{ord}}}}_{{\mathfrak m}}\) and invertibly on \(X^{{{\mathrm{ord}}}}_{{\mathfrak m}'}\) for all \({\mathfrak m}' \ne {\mathfrak m}\).
Now, to form a basis of \(X^{{{\mathrm{ord}}}}_{\mathfrak m}\), we can simply follow the method of Sect. 3.5 as long as we can produce sufficiently random symbols in \(X^{{{\mathrm{ord}}}}_{\mathfrak m}\). To do this, we form a random symbol \(\Phi \in X^{{{\mathrm{ord}}}}\) and then iterate the Hecke operator \(\prod _{{\mathfrak m}' \ne {\mathfrak m}} f_{{\mathfrak m}',\ell }(T)\) which results in projecting \(\Phi \) to the subspace \(X^{{{\mathrm{ord}}}}_{\mathfrak m}\), as desired.
Again, we mention that this method also works to produce a basis of \((X^{{{\mathrm{ord}}}}_{{\mathfrak m}})^\pm \).
4 Explicit computations with families of OMSs
In Sect. 3, we described methods of computing with ordinary families of overconvergent modular symbols. However, this discussion was all carried out on a theoretical level as a single \(\Phi \in {{\mathrm{Symb}}}_{\Gamma _0}({\mathbf {D}}^0 \hat{\otimes }\Lambda )\) is determined by an infinite amount of information. In order to compute with these families in practice, one must have a systematic method of approximating each \(\Phi \) with a finite amount of data. Moreover, such approximations must be respected by the Hecke operators. In what follows, we describe our method of approximating families of overconvergent modular symbols. Further, we verify that the methods we described in the previous section still carry through with our approximated families.
4.1 Finite approximation modules in families
We begin by reviewing the methods of [28] where a systematic method of approximating elements of \({\mathbf {D}}_k\) was given which was compatible with the \(\Sigma _0(p)\)-action. These approximations allowed for explicit computations to be carried out in the space \({{\mathrm{Symb}}}_{\Gamma _0}({\mathbf {D}}_k)\).
We seek to generalize this construction to the case of families; that is, we seek a \(\Sigma _0(p)\)-stable filtration on \({\mathbf {D}}^0 \hat{\otimes }\Lambda \). One could hope to define a nice filtration on \({\mathbf {D}}^0 \hat{\otimes }\Lambda \) by simply extending the above filtration on \({\mathbf {D}}^0\) by \(\Lambda \)-linearity. However, this filtration is not preserved by the \(\Sigma _0(p)\)-action defined in Sect. 2.5. Indeed, the \(\Sigma _0(p)\)-action on \({\mathbf {D}}^0 \hat{\otimes }\Lambda \) is defined by combining the \({\mathbf A}\)-action on \({\mathbf {D}}\) with a weight 0 action. However, \({{\mathrm{Fil}}}^M({\mathbf {D}}^0)\) is not preserved under the \({\mathbf A}\)-action. For instance, multiplication by the element z maps \({{\mathrm{Fil}}}^M({\mathbf {D}}^0)\) into \({{\mathrm{Fil}}}^{M-1}({\mathbf {D}}^0)\).
We now turn to Theorem 2.15 to see how far these power series are from being in \({\mathbb Z}_p[[pw]]\). Indeed, this theorem tells us that such power series are in \(S_z\), and thus their j-th coefficients have valuation at least \(c_p j\).
Lemma 4.1
The weight k action of \(\Sigma _0(p)\) on \({\mathbf {D}}\) preserves \(\widetilde{{{\mathrm{Fil}}}}^M({\mathbf {D}}^0)\).
Proof
Lemma 4.2
The action of \(S_z \subseteq {\mathbf A}\) on \({\mathbf {D}}\) preserves \(\widetilde{{{\mathrm{Fil}}}}^M({\mathbf {D}}^0)\).
Proof
Lemma 4.3
We have that \(\widetilde{{{\mathrm{Fil}}}}^M( {\mathbf {D}}^0 \hat{\otimes }\Lambda )\) is preserved by the \(\Sigma _0(p)\)-action.
Proof
Remark 4.4
4.2 Handling denominators
Lemma 4.5
- (1)
\(\widetilde{K}_{0}(R)\) is a \(\Sigma _0(p)\)-module;
- (2)
\(p^{M} \widetilde{{\mathcal K}}_{0}(R) \cap {\mathbf {D}}^0\hat{\otimes }R^0= \widetilde{{{\mathrm{Fil}}}}^M({\mathbf {D}}^0\hat{\otimes }R^0).\)
Proof
Part one follows exactly as in Lemma 4.3. Part two follows immediately from the definitions. \(\square \)
4.3 Solving the difference equation in \(\widetilde{{\mathcal F}}(M)\)
We now use the description of \(\widetilde{{\mathcal F}}(M)\) given in Sect. 4.2 to explain how one solves the difference equation in these approximation modules. We first review the case of a fixed weight, and then discuss the case of families.
4.3.1 The case of a fixed weight
Lemma 4.6
Proof
\(\square \)
Corollary 4.7
Proof
Lift \(\overline{\nu }\) to some element \({\nu }\) in \({\mathbf {D}}^0\) with total measure 0. Solving the difference [28, Theorem 4.5], then yields \({\mu } \in {\mathcal D}^\dag \) with \({\mu } \big | \Delta = {\nu }\). Then by Lemma 4.6, we have \(\mu \in {\mathbf {D}}^0+ p^M {\mathcal K}_0\). Projecting \(\mu \) to \(({\mathbf {D}}^0+ p^M {\mathcal K}_0)/ p^M {\mathcal K}_0 \cong {\mathbf {D}}^0/ {{\mathrm{Fil}}}^M({\mathbf {D}}^0)\) then yields a solution to the difference equation in the finite approximation module. \(\square \)
We note that the above corollary tells us the existence of solution to the difference equation in \({\mathcal F}(M)\). We now describe how to explicitly write down such a solution. Moreover, by analyzing this explicit solution, we will see that a smaller power of p is needed to control denominators.
To start, we note that the solution to the difference equation in \({\mathcal F}(M)\) is not unique.
Lemma 4.8
If \(\mu \in {{\mathrm{Fil}}}^{M-1}({\mathbf {D}}^0)\), then \(\mu \big | \Delta \in {{\mathrm{Fil}}}^M({\mathbf {D}}^0)\).
Proof
Proposition 4.9
Proof
Next, let \(\nu \) denote any lift of \(\overline{\nu }\) to \({\mathbf {D}}^0\) with total measure 0, and let \(\mu \in {\mathcal D}^\dag \) be the unique distribution satisfying \(\mu \big | \Delta = p^m \nu \) (by [28, Theorem 4.5]). Then the image of \(\mu \) in \({\mathcal F}(M-1)\) equals \(\overline{\mu }_0\) since the explicit formulas in [28, Theorem 4.5] exactly match the formulas defining \(\overline{\mu }_0\) in this proposition. (Note that our choice of m allows us to form this projection.) Thus, for any \(\overline{\mu } \in {\mathcal F}(M)\) lifting \(\overline{\mu }_0\), we have that the image of \(\mu \) in \({\mathcal F}(M)\) equals \(\overline{\mu }\) up to some distribution taking values in \({{\mathrm{Fil}}}^{M-1}({\mathbf {D}}^0)\). Our proposition then follows from Lemma 4.8. \(\square \)
4.3.2 The difference equation in \(\widetilde{{\mathcal F}}(M)\)
We now generalize the discussion of the previous section to the case of families.
Lemma 4.10
Proof
Thus, by scaling our distributions by a small power of p we will be able to solve the difference equation in these finite approximation modules.
Corollary 4.11
Proof
The proof follows verbatim as in Corollary 4.7. \(\square \)
Just as in the fixed weight case, the solution to the difference equation is not unique in \(\widetilde{{\mathcal F}}(M)\). Unfortunately, the analogue of Lemma 4.8 (which describes part of the kernel of \(\Delta \)) does not quite hold in families. We instead just state a slightly weaker version of Proposition 4.9 in families.
Proposition 4.12
Proof
Next, let \(\nu \) denote any lift of \(\overline{\nu }\) to \({\mathbf {D}}^0\hat{\otimes }R^0\) with total measure 0, and let \(\mu \in {\mathcal D}^\dag (R)\) be the unique distribution satisfying \(\mu \big | \Delta = p^m \nu \) (by Lemma 3.3). Then the image of \(\mu \) in \(\widetilde{{\mathcal F}}(M-1)\) equals \(\overline{\mu }\) since the explicit formulas defining \(\mu \) exactly match the formulas defining \(\overline{\mu }\) in this proposition. By Lemma 4.10, \(\mu \in {\mathbf {D}}^0\hat{\otimes }R^0+ p^{M-1} \widetilde{{\mathcal K}}_0(R)\), and thus projecting to \(\widetilde{{\mathcal F}}(M-1)\) gives the desired result. \(\square \)
4.4 The ordinary subspace of \({{\mathrm{Symb}}}_{\Gamma _0}(\widetilde{{\mathcal F}}(M))\)
Since \(\widetilde{{\mathcal F}}(M)\) is defined by taking the reduction modulo various powers of p, the space \({{\mathrm{Symb}}}_{\Gamma _0}(\widetilde{{\mathcal F}}(M))\) has the potential to have a complicated structure even as a \({\mathbb Z}_p\)-module. However, if we restrict to the ordinary subspace, the following proposition proves that passing to the M-th approximation module is equivalent to reducing modulo \(p^M\).
Proposition 4.13
Proof
The remainder of the proposition all follows formally from the first claim and the fact that \({{\mathrm{Symb}}}_{\Gamma _0}({\mathbf {D}}^0 \hat{\otimes }\Lambda )\) is a free \(\Lambda \)-module. \(\square \)
4.5 Vector of total measures
Proposition 4.14
The map \(\alpha _{M,L}\) is injective.
Proof
By Proposition 4.13, \({{\mathrm{Symb}}}_{\Gamma _0}(\widetilde{{\mathcal F}}(M,L))^{{{\mathrm{ord}}}}\) is simply the reduction of \(X^{{{\mathrm{ord}}}}\) modulo \((p^M,w^L)\Lambda \). Likewise, \(\alpha _{M,L}\) is simply the reduction of \(\alpha \) (from Sect. 3.4) modulo \((p^M,w^L) \Lambda \). Thus, if K is the kernel of \(\alpha _{M,L}\), by Proposition 3.10, we have \(K \otimes \Lambda / {\mathfrak m}= 0\). Thus, \(K = 0\) and \(\alpha _{M,L}\) is injective. \(\square \)
Corollary 4.15
We have \(\left\{ \Phi _1^{{{\mathrm{ord}}}}, \ldots , \Phi _j^{{{\mathrm{ord}}}} \right\} \) is the start of \(\Lambda / (p^M,w^L) \Lambda \)-basis of \({{\mathrm{Symb}}}_{\Gamma _0}(\widetilde{{\mathcal F}}(M,L))^{{{\mathrm{ord}}}}\) if and only if \(\left\{ \alpha _{1,1}(\Phi _1^{{{\mathrm{ord}}}}), \ldots , \alpha _{1,1}(\Phi _j^{{{\mathrm{ord}}}}) \right\} \) is a linearly independent set in \((\Lambda / (p,w) \Lambda )^t \cong {\mathbb F}_p^t\).
4.6 Characteristic polynomials of Hecke operators
The method of Sect. 3.5 to form a basis \(X^{{{\mathrm{ord}}}}\) was to produce “random” elements of \(X^{{{\mathrm{ord}}}}\) until one was the start of a \(\Lambda \)-basis of the space. Then produce random elements until one has two elements forming the start of a \(\Lambda \)-basis. Continue this until we have a full basis (whose size we know as in footnote 3).
To carry this method out in \({{\mathrm{Symb}}}_{\Gamma _0}(\widetilde{{\mathcal F}}(M,L))^{{{\mathrm{ord}}}}\), we note that we can form elements in \({{\mathrm{Symb}}}_{\Gamma _0}(\widetilde{{\mathcal F}}(M,L))\) as described in Sect. 3.2. Note that this requires solving the difference equation in \(\widetilde{{\mathcal F}}(M,L)\) which is done explicitly in Proposition 4.12. To form elements of \({{\mathrm{Symb}}}_{\Gamma _0}(\widetilde{{\mathcal F}}(M,L))^{{{\mathrm{ord}}}}\) one then just needs to iterate the \(U_p\)-operator.^{5}
Further, to determine when elements of \({{\mathrm{Symb}}}_{\Gamma _0}(\widetilde{{\mathcal F}}(M,L))^{{{\mathrm{ord}}}}\) are the start of a \(\Lambda / (p^M,w^L) \Lambda \)-basis, we can invoke Corollary 4.15 and examine the associated vectors of total measures modulo \((p,w)\Lambda \). If these vectors are linearly independent over \({\mathbb F}_p\), then the original elements are the start of a basis.
5 Data and examples
In this section, we describe some sample computations that we have carried out with the algorithms implemented as part of this project. Specifically, we include computations of formal q-expansions of Hida families, the structure of Hida–Hecke algebras, L-invariants of modular forms and their symmetric squares, and two-variable p-adic L-functions of Hida families. For the sake of presentation, we have elided some of the data in this section; the full data is presented in Appendix 7.
We briefly indicate about how much time and space each example took to compute on a ‘member server’ on the SageMathCloud in Sage 5.11. These are meant to be ballpark estimates; for instance, we ran Example 5.2 several times taking between 6 minutes and 10 minutes with this difference attributed mostly to the varying load of the server.
5.1 Examples of q-expansions in families
If \(\mathcal {F}\) is a normalized (i.e. \(a_1=1\)) eigenform, then the \(a_n(k)\) are determined by the \(a_\ell (k)\), for \(\ell \) prime, using the standard Hecke operator recurrence relations. We may thus obtain the q-expansion from knowing the \(a_\ell (k)\). Furthermore, \(a_\ell (k)\) is the Hecke eigenvalue of the Hecke operator \(T_\ell \). Thus, if \(\Phi \) is a family of Hecke eigensymbols, we may compute the corresponding \(a_\ell \) by comparing \(\Phi |T_\ell \) with \(\Phi \).
More precisely, since \(a_\ell (k)\) is an Iwasawa function, there exist \(A_\ell (W)\in {\mathbb Z}_p\llbracket W \rrbracket \) such that \(a_\ell (k)=A_\ell ((1+p)^k-1)\). Comparing \(\Phi |T_\ell \) with \(\Phi \) directly yields \(A_\ell (W)\), and then a simple substitution yields \(a_\ell (k)\). In fact, our computations take place in the larger ring \({\mathbb Z}_p\llbracket w \rrbracket = {\mathbb Z}_p\llbracket W/p \rrbracket \). However, by Theorem 3.9, we know that the eigenvalues \(A_\ell (w)\) must land in the subring \({\mathbb Z}_p\llbracket W \rrbracket \).
Below, we provide the first few \(a_\ell \) for some examples of families passing through specific newforms. See Sects. 3.5 and 3.6 for the method used to isolate these examples. We remind the reader that specializing the variable k below to a specific non-negative integer \(k_0\) gives modular forms of weight \(k_0+2\).
Example 5.1
Example 5.2
This example took about 7 min: computing the eigenfamily took about 5.5 min and computing the 5 eigenvalues took about 1.5 min. Computing the modular symbol space and the eigenfamily used about 55 MB while the eigenvalues used about 45 MB.
Example 5.3
This example took about 11 min: 7 min for the eigenfamily and 4 min for the five eigenvalues. The eigenfamily computation used 45 MB and the eigenvalues used 120 MB.
Example 5.4
This example took about 58 min: 35 min for the eigenfamily and 23 min for the eight eigenvalues. The eigenfamily computation used 77 MB and the eigenvalues used 719 MB.
Example 5.5
Example 5.6
5.2 The structure of Hida algebras
Here, we summarize some computations of the structure of the connected components of Hida algebras.
Example 5.7
Let \(p=3\) and \(N=11\). In this case, there are two 3-ordinary cuspforms in any even weight. In weight 2, one of these forms comes from the ordinary 3-stabilization of the cuspform associated to \(X_0(11)\). The other form is the unique newform of level 33, and, moreover, these two forms admit a congruence modulo 3. Note then \((X^{{{\mathrm{ord}}}})^-\) is 2-dimensional with \((X^{{{\mathrm{ord}}}}_{\mathfrak m})^- \simeq (X^{{{\mathrm{ord}}}})^-\) making this example fundamentally different from the examples in Sect. 5.1 where we were always able to cut down to a 1-dimensional space.
Nonetheless, we know that \({\mathbb T}_{\mathfrak m}\) is a \(\Lambda \)-algebra of rank 2 and we seek to understand its structure. From a geometric perspective, possibilities for \({{\mathrm{Spec}}}({\mathbb T}_{\mathfrak m})\) include two copies of weight space glued to together at a finite collection of points (possibly only at the point of characteristic p) or a ramified cover of weight space ramified at finitely many points.
As a check, we computed \(d_\ell (W)\) for all primes \(\ell < 11\). In each case, \(d_\ell (W)\) had \(\lambda \)-invariant equal to 1, and its unique root \(\alpha _\ell \) was congruent to \(\alpha _2\) modulo the precision of the computation.
Computing the basis of the two-dimensional \((X^{{{\mathrm{ord}}}}_{\mathfrak m})^-\) took 27.5 minutes and 107MB, while computing the Hecke polynomials for primes \(\le 11\) took 33.5 minutes and 462MB.
Example 5.8
Let \(p=37\) and \(N=1\). This example gains its fame from the fact that 37 is an irregular prime with \(B_{32}\) having positive valuation at 37. In particular, there is a cuspform f of weight 32 congruent to the Eisenstein series \(E_{32}\) modulo 37. For this reason we consider the 30th component of weight space (corresponding to the classical weight 32). On this component there are exactly three ordinary normalized eigenforms: f, \(E_{32}^{{{\mathrm{ord}}}}\) (the ordinary 37-stabilization of \(E_{32}\)), and a third form not congruent to either f or \(E_{32}^{{{\mathrm{ord}}}}\).
As a check, we computed \(d_\ell (W)\) for \(\ell \le 11\) and in each case \(\lambda (d_\ell )=2\) and \(\alpha _2\) was a root of \(d_\ell (W)\) modulo our precision.
5.3 L-invariants
We collect some values of L-invariants of modular forms as well as their trace-zero adjoints. Again, full data is available in Appendix 7.2.
Example 5.9
Example 5.10
Example 5.11
5.4 Two-variable p-adic L-functions
A p-ordinary eigenform f of classical weight \(k\ge 2\) has a p-adic L-function \(L_p(s,f)\) attached to it following the work of Manin, Amice–Vélu, and Višik. Varying the form p-adically in a Hida family one can expect to ‘glue’ the one-variable functions together to obtain a two-variable p-adic L-function \(L_p(s,\kappa )\) where \(\kappa \) is a weight variable around a neighborhood of k. That this is the case is due to Ohta (unpublished), Mazur–Kitagawa [24], and Greenberg–Stevens [18]; it was a fundamental ingredient in the latter’s proof of the Mazur–Tate–Teitelbaum conjecture. Greenberg has conjectured that the generic order of vanishing of \(L_p(s,\kappa )\) along the line \(s=\kappa /2\) is at most one (and congruent to the sign of the functional equation of \(L_p(s,f)\) modulo 2) (see [27, p. 439] for this statement and for some important consequences of its proof). Additionally, Greenberg and Stevens end the introduction to [18] by asking about the linear factors of the leading term in the expansion of \(L_p(s,\kappa )\) about \(s=1\) and \(\kappa =2\) when the sign is \(-1\).
In this section, we include a few sample computations of two-variable p-adic L-functions through overconvergent modular symbols. To motivate these computations, we quickly review the single variable case. In [30, Theorem 8.3], Glenn Stevens gives a construction of the p-adic L-function of an eigenform f solely in terms of its corresponding overconvergent modular eigensymbol \(\Phi _f\), the unique overconvergent eigensymbol with the same system of eigenvalues as f. To form the p-adic L-function of f, one simply takes \(L_p(f):=\Phi _f(\{\infty \}-\{0\})\) which is a distribution on \({\mathbb Z}_p\) and restricts this distribution to \({\mathbb Z}_p^\times \). Now if \(\Phi \in {{\mathrm{Symb}}}_{\Gamma _0}({\mathbf {D}}^0 \hat{\otimes }\Lambda )\) is a family of overconvergent eigensymbols, we analogously define the two-variable p-adic L-function \(L_p(\Phi )\) to be the restriction of \(\Phi (\{\infty \}-\{0\})\) to \({\mathbb Z}_p^\times \). The result is a family of distributions on \({\mathbb Z}_p^\times \) whose specialization to any weight is the p-adic L-function of the corresponding eigenform in that weight.
In [28, Section 9], the penultimate author and Stevens explain how in practice one can compute single variable p-adic L-functions from overconvergent modular eigensymbols. The same method, which we describe now, applies in our case and allows us to compute two-variable p-adic L-functions attached to eigenfamilies of overconvergent modular symbols.
Lemma 5.12
- (1)If \(j\le n\), then$$\begin{aligned} v_p(c^{(n)}_j)\ge -\left\lfloor \frac{n}{p-1}\right\rfloor -j, \end{aligned}$$
- (2)if \(j>n\), then$$\begin{aligned} v_p(c^{(n)}_j)\ge -\left\lfloor \frac{n}{p-1}\right\rfloor -n-\left\lfloor \frac{j}{p}\right\rfloor . \end{aligned}$$
Proof
For \(j>n\), note that the sum of the valuations occurring above is bounded by the same sum where there is no restriction on the length of the partition. Of all partitions of j, the one maximizing the sum of the \(v_p(a_k)\) is \(j=p+p+\cdots +p+r\) with \(0\le r<p\). Indeed, it is clearly optimal for the parts of the partition to be powers of p and, for \(r>1\), the part \(p^r\) contributes r while the sum \(p^{r-1}+\cdots +p^{r-1}\) (p times) contributes \(p(r-1)>r\). \(\square \)
With this lemma in hand, we computed several examples of two-variable p-adic L-functions. Our code produces a power series F(T, w) with T the cyclotomic variable and w the same weight variable as above. In these examples we have made the following normalizations. As throughout the whole paper, the weight variable k is normalized to correspond to modular forms of weight \(k+2\) and is obtained by substituting \(w=((1+p)^k-1)/p\). The cyclotomic variable s is shifted by 1 so that \(s=0\) corresponds to the central point of the L-function of a weight two modular form; it is obtained by substituting \(T=(1+p)^s-1\). Furthermore, the p-adic L-function we compute is only well-defined up to a unit power series in w and we normalize it so that the first non-zero coefficient in T is a power of p times a power of w.
Example 5.13
Given the family of overconvergent modular symbols \({\mathcal F}_{11}\), computing the p-adic L-function took 30 s and 3 MB.
Example 5.14
Given the family of overconvergent modular symbols, computing the p-adic L-function took 174 s and 66 MB.
Example 5.15
Given the family of overconvergent modular symbols, computing the p-adic L-function took 45 s and 24 MB.
Here, and throughout the paper, we are normalizing the weight variable k to correspond to forms in \(M_{k+2}(\Gamma )\).
To work with other discs in weight space, one needs to replace \({\mathbf {D}}\) with smaller spaces of distributions such as \({\mathbf {D}}[r]\) with \(r<1\).
The \({\mathbb Z}_p\)-rank of \({{\mathrm{Symb}}}_{\Gamma _0}({{\mathrm{Sym}}}^k({\mathbb Z}_p^2))^{{{\mathrm{ord}}}}\) is simply given by the number of non-zero roots of the characteristic polynomial of \(U_p\) acting on \({{\mathrm{Symb}}}_{\Gamma _0}({{\mathrm{Sym}}}^k({\mathbb F}_p^2))\).
We note that this polynomial also arises as the characteristic polynomial of T acting on the space of p-ordinary modular symbols of weight k defined over \({\mathbb F}_p\) for any \(k \equiv m \pmod {p-1}\) and is thus readily computed.
To verify if a symbol \(\Phi \) is actually in the ordinary subspace, one looks at \(\Phi \), \(\Phi | U_p\), \(\Phi |U_p^2\), \(\ldots \), until there is a relation.
Declarations
Open Access
This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Acknowledgements
We would like to thank the Southwest Center for Arithmetic Geometry for organizing the 2011 Arizona Winter School where the work on this article began as a student project. We would also like to thank the participants of Sage Days 44 for their work in porting the original Sage scripts into a full blown Sage package. We would like to thank Sage, as well as the SageMath Cloud, where we developed the algorithms and computed the examples in this article. We would like to thank Glenn Stevens for his support of this project and Frank Calegari for some very helpful conversations. Finally, our thanks go to the referee for some comments and suggestions that improved the clarity of this article.
Robert Harron was supported by NSA Grant #H98230-13-1-0223 during part of this project. Robert Pollack was supported by NSF Grant DMS-1303302.
Authors’ Affiliations
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