Questions tagged [modular-group]

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Fourier expansion of half-integral weight Eisenstein series associated with Kohnen's plus space

The Eisenstein series associated with Kohnen's plus space in $\Gamma_{0}(4)$ is expressed as follows, \begin{align} \begin{split} E_{k + \tfrac{1}{2}}^{\infty}(\tau) =& \sum\limits_{\...
Spoilt Milk's user avatar
2 votes
0 answers
72 views

Is the continued fraction of a constructible number special in some way?

Rationals have finite CF and quadratic have periodic CF. CF in turn can be represented in terms of the modular group SL2(Z), e.g. using the standard generators S(z)=-1/z and T(z)=z+1. On the other ...
Lucian Ionescu's user avatar
2 votes
0 answers
87 views

$\mathrm{GL}(n, \mathbb{Z})$-equivariant maps on $\mathrm{GL}(n, \mathbb{R})$

$\DeclareMathOperator\GL{GL}$Can you describe the maps from $\GL(n, \mathbb{R})$ to $\GL(n, \mathbb{R})$ that are equivariant w.r.t. right multiplication by $\GL(n, \mathbb{Z})$? I'm interested even ...
gm01's user avatar
  • 325
3 votes
1 answer
217 views

Irreducible unitary representation of PSL(2,Z)

Do we already know the classification of the finite-dimensional irreducible unitary representations of the modular group $PSL(2,\mathbb{Z})=\mathbb{Z}/2*\mathbb{Z}/3$? I'm particularly interested in ...
Leo's user avatar
  • 541
2 votes
1 answer
436 views

Takesaki II proposition 3.15 proof about modular automorphism groups: mistake in book?

Consider the following fragment from Takesaki's second volume "Theory of operator algebras" in chapter VIII "Modular automorphism groups" p122-123. Why is it possible to choose an ...
Andromeda's user avatar
  • 209
2 votes
1 answer
143 views

Fractional group index?

The Hecke group of level two, $\Gamma_{0}(2)$, is an index-$2$ subgroup of the Fricke group of level two, $\Gamma_{0}^{+}(2)$, i.e. $\left[\Gamma_{0}^{+}(2):\Gamma_{0}(2)\right] = 2$. The index of $\...
Spoilt Milk's user avatar
1 vote
0 answers
71 views

Fourier series with fractional linear transformation of argument

Set as usual $e(z):=\exp(2\pi iz)$. Having two series that are related for all $z$ in the upper halfplane via a transformation formula $$ \sum_{n=1}^{\infty}{\alpha_n e\Big(n\frac{az+b}{cz+d}\Big)}=p(...
Marcus's user avatar
  • 396
1 vote
0 answers
94 views

Different modular data with same T-matrix

Modular data (MD) is an invariant of a modular fusion category $\mathcal{C}$. It is a couple of symmetric matrices $S, T \in M_r(\mathbb{C})$ with: $r$ the rank of $\mathcal{C}$, $S$ invertible, $T$ ...
Sebastien Palcoux's user avatar
1 vote
0 answers
75 views

continued fractions and cusp non-excursions

Consider the modular surface $X:=\mathbb{H^2}/PSL_2(\mathbb{Z})$. Fix a width-of-cusp parameter $w, 0<w<<1$. Let $B_w$ be the cusp neighborhood of width $w$. (So $w=1$ corresponds to the ...
Kevin M Pilgrim's user avatar
26 votes
4 answers
1k views

Equations defining hyperbolic geodesics in $\mathbb C \setminus\{0,1\}$

Let $X=\mathbb C\setminus\{0,1\}$, equipped with the hyperbolic structure it inherits from Klein's modular $\lambda$ function $\lambda:\mathbb H \to X$. In each (non-peripheral and nontrivial) free-...
Jonah Gaster's user avatar
6 votes
1 answer
384 views

Mapping class group of torus, why is $(ST)^3=S^2$?

In the context of topological quantum field theories, I am interested in the mapping class group of a torus. Here I can consider the torus as a square with identified edges and also decorated with ...
as2457's user avatar
  • 295
3 votes
2 answers
284 views

A question about congruence subgroups [closed]

For which $N_1$ and $N_2$ and $N$ be the greatest common divisor of $N_1$ and $N_2$, it is true that a congruence subgroup in $\mathrm{SL}_2(\mathbb{Z})$ generated by $\Gamma(N_1)\cup\Gamma(N_2)$ ...
hyy qhh's user avatar
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0 answers
107 views

Coset representatives of a principal congruence subgroup by another principal congruence subgroup

Consider the principal congruence subgroup $\Gamma(N)$, this consists of entries in $\mathrm{SL}_2(\mathbb{Z})$ congruent to the identity modulo $N$. Consider another principal congruence subgroup $\...
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2 votes
1 answer
247 views

When does the double coset representative for a congruence subgroup contain a $\text{SL}_2(\mathbb{Z})$-conjugacy class?

In the paper p-adic L-functions and p-adic periods of modular forms, Greenberg/Stevens assert that if $\sigma_l:=\begin{pmatrix}l&0\\0&1\end{pmatrix}$ is the usual Hecke operator at $l$ double ...
xir's user avatar
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3 votes
1 answer
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How do modular functions of level $N>1$ transform under the full modular group?

Let $f$ be a modular function of level $N>1$ and let $\gamma\in SL_2(\mathbf Z)$. Write $$f(\gamma\tau)=j(\gamma,\tau)f(\tau).$$ First question What can we say in general about the factor $j(\...
Shimrod's user avatar
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3 votes
1 answer
112 views

Representatives for the action on an unknown set of matrices by an unknown modular subgroup

Let $\Gamma$ be the modular group and $\mathcal M_n$ the set of all primitive matrices with determinant $n\geq1$. Recall that a primitive matrix has relatively prime entries. The modular group $\...
Shimrod's user avatar
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6 votes
1 answer
427 views

Behavior of a modular form in the lower strip

Let $f$ be an (elliptic) modular form of weight $k>0$, and consider the vertical strip $S_m=\{x+iy\in\mathbb{C}:|x|\le 1/2, y>m$}. For every $m\ll 1$, the fundamental domain for $SL_2(Z)$ is ...
Angelo Rendina's user avatar
3 votes
1 answer
416 views

The degree of the cube root of the $j$-invariant

I have a question which is fairly elementary, but first I must provide relevant context. Without it, my question would seem rather arbitrary and scarcely interesting. Note also that my question can be ...
Shimrod's user avatar
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5 votes
0 answers
180 views

Modular functions of the type $\mathfrak f(\cdot)^{k}\mathfrak f(\cdot)^{23nk}$

Let $\eta(\omega)$ be the Dedekind eta function and let $$\mathfrak f(w)=e^{-\pi i/24}\frac{\eta((\omega+1)/2)}{\eta(\omega)}.$$ In his paper On the “gap” in a theorem of Heegner, Stark fills the gap ...
Shimrod's user avatar
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6 votes
1 answer
218 views

Current interest in geometric properties of Hilbert fundamental domains

Harvey Cohn published several articles in the 1960's analyzing geometric properties of fundamental domains for Hilbert modular surfaces. H. Cohn, "On the shape of the fundamental domain of the ...
j0equ1nn's user avatar
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11 votes
1 answer
2k views

Does every Coxeter group arise from a BN-Pair? Does $\text{PGL}_2(\Bbb{Z})$?

The question is in the title. Maybe I should explain my interest in it though. To every Coxeter group $(W,S)$ (and even more general groups) and a system of parameters $(a_s,b_s)_{s \in S}$ one can ...
Nicolas Schmidt's user avatar
6 votes
1 answer
479 views

Fourth cohomology of the modular group

Is $H^4(PSL(2,\mathbb{Z}),\mathbb{Z})$ known? I ask this in response to the recent calculation of the same cohomology group for $\mathrm{Co}_0$ and $\mathrm{Co}_1$.
David Roberts's user avatar
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15 votes
4 answers
1k views

Subgroups of $SL_2(\mathbb R)$ which contain $SL_2(\mathbb Z)$ as a finite index subgroup

Let $G\subset \mathrm{SL}_2(\mathbb R)$ be a subgroup such that $\mathrm{SL}_2(\mathbb Z)\subset G$. What are the possible groups such that $\mathrm{SL}_2(\mathbb Z)\subset G$ is of finite index? Is $...
Honing's user avatar
  • 195
2 votes
1 answer
111 views

Compute the automaton for the modular group

The modular group $\mathrm{PSL}_2(\mathbb{Z})$ has 3 generators $A,B,C$, where $$A:z\to z+1,\quad B:z\to z-1,\quad C:z\to -1/z.$$ I want to compute the automaton that recognize the words of the ...
zemora's user avatar
  • 545
3 votes
3 answers
539 views

Finite subgroups (not finite index, just finite) of the modular group

The modular group is commonly described as the group of linear fractional transforms $z \mapsto \displaystyle \frac{az+b}{cz+d}$ with $a,b,c,d$ integers and $ad-bc = 1$. Of course, a great deal is ...
Gregory Dresden's user avatar
14 votes
2 answers
579 views

Non-congruence normal subgroups of $SL_2(\mathbb{Z}[1/2])$

Let $G=SL_2(\mathbb{Z}[1/2])$, i.e., the modular group (if you wish) over the ring $\mathbb{Z}[1/2]$ consisting of rationals whose denominators are powers of $2$. Unlike $SL_2(\mathbb{R})$, $G$ is ...
H A Helfgott's user avatar
  • 19.3k
5 votes
1 answer
267 views

Upper bound on level of a congruence subgroup of the modular group

Let $\Gamma = PSL(2,\mathbb{Z}) = \langle S,T \ | \ S^2=(ST)^3=1 \rangle$. Let $G$ be some mystery normal subgroup of $\Gamma$ that we happen to think may be congruence. Recall that a subgroup of $\...
Joseph Ricci's user avatar
20 votes
0 answers
823 views

In what sense is the braid group $B_3$ the universal central extension of the modular group $\Gamma$?

First let's recall some definitions. Let $G$ be a perfect group, so that $$H^2(G, A) \cong \text{Hom}(H_2(G), A)$$ for all abelian groups $A$ by universal coefficients. This means that when $A = ...
Qiaochu Yuan's user avatar
2 votes
0 answers
83 views

Action of a Drinfeld modular group on a Drinfeld symmetric space

Let $\Bbb C_\infty$ be functional field case analog of $\Bbb C$, i.e. $\Bbb C_\infty$ is the completion of the algebraic closure of the field of Laurent series $\Bbb F_q((\theta^{-1}))$, where $q$ is ...
Dmitry Logachev's user avatar
1 vote
2 answers
202 views

Counting number of $2\times 2$ unimodular matrices of particular type

From set of numbers from $\Bbb S=\{0,1,\dots,m\}$, how many distinct $3\times 3$ unimodular matrices parametrized by $(a,b,c,d,e,f)\in\Bbb S^6$ of following type can one form? \begin{bmatrix} a^2 &...
user avatar
6 votes
3 answers
769 views

Enumerating cosets of the modular group

Suppose we chose the generators $f(z) = z+1, g(z) = z-1, h(z) = -1/z$ for the modular group $\Gamma$ (i.e. the group of fractional linear tranformations $z \mapsto (az +b)/(cz+d)$ with $a,b,c,d \in \...
Pablo Lessa's user avatar
  • 4,194
11 votes
1 answer
526 views

Is there a faithful transitive locally finite action of the modular group?

Is there a faithful transitive action of $G = \mathrm{PSL}_2(\mathbb{Z})$ on $\mathbb{Z}$ such that orbits under each $g \in G$ are finite?
Pablo's user avatar
  • 11.2k
4 votes
2 answers
239 views

Images of the fundamental domain of $\text{SL}_2(\mathbb{Z})\backslash \mathbb{H}$ whose Euclidean area is large

Let $S$ be a compact subset of the closure of the upper half plane. (Assume that $S$ is a (Euclidean) rectangular box, if you wish.) Let $D$ be the standard fundamental domain of $\text{SL}_2(\mathbb{...
H A Helfgott's user avatar
  • 19.3k
3 votes
2 answers
700 views

the Action of $SL_2(\mathbb{R})$ on a fundamental domain of $\Gamma$

Let $F=\{z\in H: |z|\geq1, |Re(z)|\leq 1/2\}$. It is a fundamental domain of the modular group $\Gamma$ acting on the upper half plane $H$. (Strictly speaking, one should take part of the boundary ...
user45392's user avatar
  • 125
10 votes
4 answers
1k views

Analytic function avoiding elements of the modular group

A friend recently told me the following two facts, for which he cannot recall a proof or a reference (but he remembers seeing them in the literature): Let $f$ be a holomorphic function mapping the ...
Alexandre Eremenko's user avatar
7 votes
6 answers
2k views

Are there noncongruence subgroups (of finite index) of the modular group generated only by 2 or 3 elements?

By the modular group I mean either $SL(2,\mathbb{Z})$ or $PSL(2,\mathbb{Z})$. Where can I find examples of these? Another question: is there a good (ideally analytical, but possibly computer-aided) ...
Will Chen's user avatar
  • 10k
7 votes
4 answers
2k views

Does every polynomial diophantine equation have solutions modulo p?

Obviously, this is not exactly true; what I am really asking is whether any diophantine polynomial equation with integer coefficients (let's call them DPEICs) who's solution does not admit ...
IronBeard's user avatar
3 votes
1 answer
359 views

Simplifying presentations of modular subgroups

I've been using the Reidemeister-Schreier process (detailed in e.g. Holt et al. - Handbook of Computational Group Theory) to find the presentations of various modular subgroups. For example, this ...
Jimeree's user avatar
  • 383
8 votes
3 answers
2k views

Are congruence subgroups of the modular group finitely presented?

Are the congruence subgroups of the modular group $\Gamma\equiv\mathrm{PSL}\left(2,\mathbb{Z}\right)$ (e.g. $\Gamma\left(n\right)$, $\Gamma_{0}\left(n\right)$, $\Gamma_{1}\left(n\right)$ etc.) ...
Jimeree's user avatar
  • 383
1 vote
2 answers
406 views

Identifying Subgroups of the Modular Group via Permutation Representations on Cosets

Suppose we have a known group G and an unknown subgroup H. The permutation representation of G on the cosets of H gives a permutation group C, which is known. Is it possible to identify the generators ...
Jimeree's user avatar
  • 383
2 votes
1 answer
979 views

A question about Ahlfors's proof of modular function being a covering space of the twice punctured plane

I have a question about Ahlfors's proof of modular function being a covering space of the twice punctured plane .See Ahlfors' complex analysis, second edition, page 272. You can either explain or ...
Analysis Now's user avatar
  • 1,451
9 votes
1 answer
698 views

Automorphisms of a matrix in Smith normal form?

Added: Amritanshu Prasad's answer makes it clear that I am really asking for a description of the group of integer unimodular matrices $P$ such that $D^{-1}PD$ is also integer. These matrices are ...
Will Orrick's user avatar
  • 2,110
11 votes
3 answers
1k views

Congruence subgroups as abstract groups

This is probably well know, and maybe even trivial, but not to me. Consider for concreteness the subgroup $$ \pm\Gamma_0(3)=\left\{\begin{pmatrix}a & b \\ c & d\end{pmatrix}:\;a,b,c,d\in\...
Alex B.'s user avatar
  • 12.8k
12 votes
4 answers
834 views

Congruence Subgroups as Open Subgroups of the Modular Group Under the Right Topology

It occurred to me that a subgroup of the modular group $\Gamma$ is a congruence subgroup iff it contains a subgroup of the form $\Gamma(N)$, while a subgroup of a general topological group is open iff ...
David Corwin's user avatar
  • 15.1k
18 votes
2 answers
2k views

Distinguishing congruence subgroups of the modular group

This question is something of a follow-up to Transformation formulae for classical theta functions . How does one recognise whether a subgroup of the modular group $\Gamma=\mathrm{SL}_2(\mathbb{Z})$ ...
Robin Chapman's user avatar