Questions tagged [quasimodular-forms]

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Logarithmic vector-valued modular functions and quasimodular forms with misleading modular weights

I have a somewhat imprecise question about functions with reasonably nice modular transformations that don't seem to fit nicely into what I understand of the plain vanilla theory of modular and ...
Christopher Beem's user avatar
2 votes
0 answers
148 views

theta function with a low bound in the sum

I encounter a sum similar to the Jacobi Theta function except there is a lower bound $-J$ with $J\geq 0$: \begin{equation} f(q,x)=\sum_{n=-J}^\infty q^{n^2}x^n. \end{equation} My question is whether ...
color's user avatar
  • 99
14 votes
2 answers
1k views

Complex Multiplication and algebraic integers

Let $q=e^{2\pi i\tau}$ and $$E_2(\tau) = 1 - 24 \sum_{n=1}^\infty\frac{nq^n}{1-q^n}$$ be the Eisenstein Series of weight $2$ and let $E_2^*(\tau) = E_2(\tau) - \frac{3}{\pi\cdot Im(\tau)}$ be the ...
L. Milla's user avatar
  • 598
6 votes
1 answer
578 views

How to compute Coefficients in Chudnovsky's Formula?

My aim is to understand all three coefficients arising in the Chudnovsky-Formula (see also Question 300385). Two of them are easily computed, but I failed with the third: It is known that for all $\...
L. Milla's user avatar
  • 598
21 votes
1 answer
1k views

Why does this quasi-modular function have integral values?

It is a well-known result that the modular function $1728J(\tau) := \frac{1728E_4(\tau)^3}{E_4(\tau)^3-E_6(\tau)^2}$ has integral values if $\tau$ has class number 1 - for example at $\tau_{163}:=\...
L. Milla's user avatar
  • 598
16 votes
0 answers
394 views

Power series which are $p$-adic modular forms for all $p$

Suppose that, for some integer $k$, a series $f(q) \in \mathbb Q \otimes \mathbb Z[[q]]$ has the property that for every prime $p$, $f(q)$ is the $q$-expansion of a $p$-adic modular form of weight $k$ ...
Bruno Joyal's user avatar
  • 3,860
4 votes
1 answer
822 views

Algebraic independence of $E_2$, $E_4$ and $E_6$

In "M. Kaneko and D. Zagier, A generalized Jacobi theta function and quasimodular forms, Prog. Math. 129, 165-172 (1995)" there is a proposition stating essentially that $E_2$, $E_4$ and $E_6$ are ...
Jonas's user avatar
  • 43
1 vote
1 answer
311 views

$f,f',...,f^{(j)}$ is $\mathbb C$-linearly independent if $f$ is a modular form

Conjecture: Let be $f$ a modular form of weight $k$ and $j$ a strictly positive integer, then the set $f,f',...,f^{(j)}$ is $\mathbb C$-linearly independent in $A$. Is that conjecture true or false? ...
MMa's user avatar
  • 53
9 votes
1 answer
712 views

How do the rings of level $N$ quasi-modular forms related to the rings of modular forms?

It is well known that the graded algebra $\mathcal{M}(1)$ of Modular forms for $\Gamma = PSL_2(\mathbb{Z})$ is the polynomial algebra $$ \mathcal{M}(1) = \mathbb{C}[E_4, E_6] $$ where $E_4$ and $E_6$ ...
Simon Rose's user avatar
  • 6,240
11 votes
2 answers
2k views

Binomial coefficients and derivatives of modular forms

Let $E_2$, $E_4$, and $E_6$ denote the standard Eisenstein series. The usual variables $q=e^{2\pi i\tau}$ allow us to regard the $E_n$'s as functions on either the upper half plane or the unit disk ...
charris's user avatar
  • 694
4 votes
2 answers
841 views

What literature is known about MacMahon's generalized sum-of-divisors function?

MacMahon in the paper Divisors of Numbers and their Continuations in the Theory of Partitions defines several generalized notions of the sum-of-divisors function; for example, if we write $a_{n,k}$ ...
Simon Rose's user avatar
  • 6,240
16 votes
3 answers
2k views

How do modular forms of half-integral weight relate to the (quasi-modular) Eisenstein series?

The Eisenstein series $$ G_{2k} = \sum_{(m,n) \neq (0,0)} \frac{1}{(m + n\tau)^{2k}} $$ are modular forms (if $k>1$) of weight $2k$ and quasi-modular if $k=1$. It is clear that given modular forms $...
Simon Rose's user avatar
  • 6,240
14 votes
2 answers
1k views

Modular equations for quasimodular forms

This problem is motivated by this question and by teaching modular polynomials for the classical modular invariant $j(\tau)$. The latter implies that if we consider the fields of modular functions $\...
Wadim Zudilin's user avatar