0
votes
1answer
16 views

Weber function takes on real values?

Why does the Weber function take on real values? In the middle of the proof, the author argues... "cleary it is real-valued." I don't follow the argument. Info about the Weber function ...
1
vote
2answers
95 views

Proving a complex function is continuous.

I've recently started complex analysis but I have very little background in complex numbers and to make sure I don't fall behind I'm doing some extra exercises one of which is Show $f$ is continuous ...
1
vote
1answer
57 views

The natural boundary of the modular $\lambda$ function is the real axis

I want to solve the following problem from Ahlfors' text: Show that the function $\lambda(\tau)$ introduced in Chap. 7, Sec. 3.4, has the real axis as a natural boundary. Here $\lambda(\tau)$ is ...
4
votes
1answer
98 views

Where does Klein's j-invariant take the values 0 and 1, and with what multiplicities?

I tried to solve the following problem from Ahlfors' text, please verify my solution: Where does the function $$J(\tau)=\frac{4}{27} \frac{(1-\lambda+\lambda^2)^3}{\lambda^2(1-\lambda)^2} $$ ...
2
votes
1answer
59 views

Why is the modular $\lambda$ function a quotient of two meromorphic functions in the U.H.P.?

In Ahlfors' complex analysis text, page 278 it says: It is quite clear from (9) that $\lambda(\tau)$ is the quotient of two analytic functions in the upper half plane $\text{Im} \tau > 0$. ...
4
votes
2answers
109 views

Topology of the completed upper-half plane

Define the topology on $\mathbb{H}^* : = \mathbb{H} ∪ \mathbb{Q} ∪\infty$ by taking a basis of open sets around $\infty$ to be $S_{\epsilon} : = \{ z ∈ H : Im ( z ) > 1 /\epsilon \}∪\infty$ , and ...
2
votes
1answer
53 views

Why the function $g(q)=f(\log(q)/(2\pi i))$ is well defined?

Let $D'=\{q\in \mathbb{C}: |q|<1\} - \{0\}$. Let $g: D' \to \mathbb{C}$ be defined by $g(q)=f(\log(q)/(2\pi i))$. Here $f: \mathcal{H} \to \mathbb{C}$ is a weakly modular function such that $f(\tau ...
4
votes
1answer
92 views

Proving convergence of a Hilbert modular theta function $\vartheta(z):= \sum\limits_{x \in \mathcal{O}_F} e^{\pi i \operatorname{Tr}(x^2 z)}$

I'm trying to understand a somewhat sketchy proof that I found online of the convergence of the analog of Jacobi's theta function $\displaystyle{\theta(\tau) := \sum_{n = -\infty}^{\infty} e^{2 \pi i ...
5
votes
0answers
142 views

When does a modular form satisfy a differential equation with rational coefficients?

Given a modular form $f$ of weight $k$ for a congruence subgroup $\Gamma$, and a modular function $t$ for $\Gamma$ with $t(i\infty)=0$, we can form a function $F$ such that $F(t(z))=f(z)$ (at least ...
1
vote
1answer
53 views

Question about a property of elliptic function

Let $E=\mathbb{C}/\Lambda$ be a complex elliptic curve where $\Lambda=\omega_1\mathbb{Z}\oplus\omega_2\mathbb{Z}$ Let $f$ be a nonconstant elliptic function with respect to $\Lambda$. Let ...
5
votes
1answer
66 views

A question about complex tori.

Let $\Lambda$,$\Lambda'$ be two lattice in $\mathbb{C}$ and $m\neq 0\in\mathbb{C}$ satisfying $$ m\Lambda\subset\Lambda' $$ The, the book I'm reading says that by the theory of finite Abelian groups ...
1
vote
1answer
69 views

Why the kernel of isogeny is finite?

It is said that the kernel of a isogeny is finite because it is discrete and complex tori are compact. I have some questions about this. 1. Following is my reason for the kernel is discrete. ...
2
votes
1answer
63 views

Divisor of a modular form

I am trying to compute the divisor of $\Delta(z)/\Delta(pz)$ on the modular curve $X_0(p)$ where $p$ is a prime. I know that as a function on the full modular group, the $\Delta$ function has only a ...
1
vote
1answer
132 views

Dimension of modular forms for a congruence subgroup using contour integration.

There is a well known proof for finding the dimension of modular forms for the full modular group using the residue formula. It is on page 71 of these online notes: ...
0
votes
0answers
215 views

Modular Form Holomorphic at Cusps

Let $f$ be a weight $k$ modular form for $\Gamma$ and $f\vert_{\sigma}(z) = f(\sigma z)(cz + d)^{-k}$ where $\sigma = \begin{pmatrix} a & b\\c & d\end{pmatrix}$. Why is it true that $f$ is ...
1
vote
1answer
214 views

Eisenstein series estimate from Stein-Shakarchi

I need help with this one from Stein & Shakarchi. Let $$E_{4}(\tau)=\sum_{(n,m)\neq (0,0)}{\frac{1}{(n+m\tau)^{4}}}$$ be the Eisenstein series of order $4$. Show that ...
0
votes
1answer
206 views

Nice formulas for the lambda invariant of an elliptic curve

Where can I find some nice formulas for the lambda invariant of an elliptic curve? I vaguely recall there's a nice product formula in terms of $q$, but a google search didn't give me much. Also, are ...
0
votes
0answers
85 views

The lambda invariant

Consider the strip $\{x+iy: -1\leq x < 1 , y>1/2\}$ in the complex upper half plane and let $\lambda$ be the usual $\Gamma(2)$-invariant modular function on the complex upper-half plane. ...
2
votes
1answer
114 views

Real elliptic curves in the fundamental domain of $\Gamma(2)$

An elliptic curve (over $\mathbf{C}$) is real if its j-invariant is real. The set of real elliptic curves in the standard fundamental domain of $\mathrm{SL}_2(\mathbf{Z})$ can be explicitly ...
1
vote
1answer
61 views

Is the image of this strip equal to the modular curve $Y(2)$

Let $B$ be the image of the strip $$\{x+iy : -1\leq x< 1, y>\frac{1}{2} \} \subset \mathbf{H}$$ in the modular curve $Y(2)$ under the quotient map $\mathbf{H} \to Y(2)$. Is $B=Y(2)$? Is ...
1
vote
1answer
169 views

Modular functions of weight zero

The following question was suggested by Sasha's answer to the following question : Is the derivative of a modular function a modular function . Question. What are the modular functions with respect ...
3
votes
1answer
147 views

Is the derivative of a modular function a modular function

Fix a positive integer $n$. Let $f:\mathbf{H}\longrightarrow \mathbf{C}$ be a modular function with respect to the group $\Gamma(n)$. Is the derivative $$\frac{df}{d\tau}:\mathbf{H}\longrightarrow ...
2
votes
1answer
84 views

Complex line integral of modular function

Let $f$ be a modular function on the upper half-plane, $\rho = e^{2\pi i/3}$ and $v_\rho(f)$ the order of $f$ at $\rho$, i.e. the integer $n$ such that $f/(z-\rho)^n$ is holomorphic and non-zero at ...
2
votes
1answer
95 views

Are there any relations between the two statements about the poles and zeros of a modular form?

I learned two pieces of info on the relations of poles and zeros of a modular form $f$ as follows: $\mathbb{H}$ is the upper half plane, $G$ is the modular group $\text{SL}_2(\mathbb{Z})$, ...
3
votes
1answer
112 views

Are there any interpretations for the weights of modular forms?

To be specific, do the weights have some geometric meanings? A modular form $f$ satisfies $f(\frac{az+b}{cz+d})(cz+d)^{-2k}=f(z)$, where $z\in \mathbb{C}$. $k$ or $2k$ is called the weight of $f$.