Structure of the algebra of mod $p$ modular forms - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-19T08:50:42Z http://mathoverflow.net/feeds/question/93059 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/93059/structure-of-the-algebra-of-mod-p-modular-forms Structure of the algebra of mod $p$ modular forms Joël 2012-04-04T01:41:44Z 2012-04-04T01:58:44Z <p>Let me first define the algebra $M$ I am talking about: let us fix a prime $p$, an integer $N$ not divisible by $p$. For $k$ an integer, let me call $N_k$ the $\mathbb{Z}$-module of modular forms of level $\Gamma_0(N)$ (or $\Gamma_1(N)$, or $\Gamma(N)$, whichever you prefer) of some weight $\leq k$, and whose $q$-expansion (at infinity) is in $\mathbb{Z}[[q]]$. Let me call $M_k \subset \mathbb{F}_p[[q]]$ the image of $N_k$ by the map $f \mapsto \tilde f$: "reduction mod $p$ of the $q$-expansion of $f$". It is clear that $M_k \subset M_{k+1}$ and let $M = \cup_{k \geq 0} M_k \subset \mathbb{F}_p[[q]]$. It is clear that $M$ is a sub-algebra of $\mathbb{F}_p[[q]]$, that one can call <em>the algebra of mod $p$ modular forms</em> of level $\Gamma_0(N)$ (or $\Gamma_1(N)$, or $\Gamma(N)$).</p> <p>When $N=1$, the structure of $M$ has been determined by Swinnerton-Dyer in his famous paper in Antwerpen III (and there is an also famous Bourbaki <a href="http://archive.numdam.org/ARCHIVE/SB/SB_1971-1972__14_/SB_1971-1972__14__319_0/SB_1971-1972__14__319_0.pdf" rel="nofollow">seminar</a> by Serre on this subject): when $p=2,3$, $M=\mathbb{F}_p[\tilde \Delta]$ and when $p\geq5$, $M=\mathbb{F}_p[\tilde E_4,\tilde E_6]/(A_p(\tilde E_4,\tilde E_6)-1)$, where $A_p$ is a polynomial in two variables, square-free, and homogeneous of degree $p-1$ if we consider $\tilde E_4$ of degree $4$ and $\tilde E_6$ of degree $6$. In particular, $M$ is in any case a noetherian domain of dimension $1$.</p> <p>My question is: </p> <blockquote> <p>In which cases $N>1$ has the structure of $M$ been determined? What is known about its structure?</p> </blockquote> <p>I have not been able to find any reference discussing this question, but such references certainly exist. (I have found many references discussing the actions of the Hecke operators on the $M_k$ and $M$, and the algebras they generate, but this is not the same question: there is no Hecke operators in the one I am asking.)</p>