When Can We Say A Theta Function Is A Modular Form?

Introduction

In the realm of complex analysis and number theory, modular forms and theta functions are two fundamental concepts that have been extensively studied. Modular forms are analytic functions on the upper half-plane that transform in a specific way under the action of the modular group, while theta functions are a type of function that can be expressed as a sum of exponentials. In this article, we will explore the relationship between theta functions and modular forms, and discuss the conditions under which a theta function can be considered a modular form.

Theta Functions

A theta function is a function of the form $\Theta(x;q) = \sum_{n\in\mathbb{Z}}(-1)^nq^{\binom{n}{2}}x^n$, where $q = e^{2\pi i\tau}$ and $\tau$ is a complex number in the upper half-plane. This function is also known as the Jacobi theta function. The Jacobi theta function is a special case of a more general class of functions known as theta functions, which can be expressed as a sum of exponentials.

Modular Forms

A modular form is an analytic function on the upper half-plane that satisfies certain transformation properties under the action of the modular group. Specifically, a function $f$ is said to be a modular form of weight $k$ if it satisfies the following properties:

• $f$ is analytic on the upper half-plane.
• $f$ satisfies the transformation property $f(\gamma z) = (cz + d)^kf(z)$ for all $\gamma = \begin{pmatrix} a & b \\ c & d \end{pmatrix} \in \mathrm{SL}_2(\mathbb{Z})$.
• $f$ has a Fourier expansion of the form $f(z) = \sum_{n=0}^{\infty} a_n e^{2\pi inz}$.

When is a Theta Function a Modular Form?

Now that we have defined both theta functions and modular forms, we can ask the question: when can we say that a theta function is a modular form? The answer to this question is not immediately obvious, and it requires a careful analysis of the properties of theta functions and modular forms.

One way to approach this question is to consider the transformation properties of theta functions under the action of the modular group. Specifically, we can ask whether a theta function satisfies the transformation property $f(\gamma z) = (cz + d)^kf(z)$ for all $\gamma = \begin{pmatrix} a & b \\ c & d \end{pmatrix} \in \mathrm{SL}_2(\mathbb{Z})$.

Transformation Properties of Theta Functions

To determine whether a theta function satisfies the transformation property, we need to compute the value of the theta function at $\gamma z$ for an arbitrary $\gamma = \begin{pmatrix} a & b \\ c & d \end{pmatrix} \in \mathrm{SL}_2(\mathbb{Z})$. Using the definition of the theta function, we can write:

$$\Theta(\gamma z;q) = \sum_{n\in\mathbb{Z}}(-1)^nq^{\binom{n}{2}}(\gamma z)^n$$

Using the fact that $\gamma z = \frac{az+b}{cz+d}$, we can rewrite this expression as:

$$\Theta(\gamma z;q) = \sum_{n\in\mathbb{Z}}(-1)^nq^{\binom{n}{2}}\left(\frac{az+b}{cz+d}\right)^n$$

Now, we can use the binomial theorem to expand the expression $(\frac{az+b}{cz+d})^n$:

$$(\frac{az+b}{cz+d})^n = \sum_{k=0}^n \binom{n}{k} \left(\frac{az+b}{cz+d}\right)^k \left(\frac{az+b}{cz+d}\right)^{n-k}$$

Substituting this expression back into the original equation, we get:

$$\Theta(\gamma z;q) = \sum_{n\in\mathbb{Z}} \sum_{k=0}^n (-1)^n q^{\binom{n}{2}} \binom{n}{k} \left(\frac{az+b}{cz+d}\right)^k \left(\frac{az+b}{cz+d}\right)^{n-k}$$

Now, we can use the fact that $\binom{n}{k} = \binom{n}{n-k}$ to rewrite the expression as:

$$\Theta(\gamma z;q) = \sum_{n\in\mathbb{Z}} \sum_{k=0}^n (-1)^n q^{\binom{n}{2}} \binom{n}{n-k} \left(\frac{az+b}{cz+d}\right)^k \left(\frac{az+b}{cz+d}\right)^{n-k}$$

Using the fact that $\binom{n}{n-k} = \binom{n}{k}$, we can rewrite the expression as:

$$\Theta(\gamma z;q) = \sum_{n\in\mathbb{Z}} \sum_{k=0}^n (-1)^n q^{\binom{n}{2}} \binom{n}{k} \left(\frac{az+b}{cz+d}\right)^k \left(\frac{az+b}{cz+d}\right)^{n-k}$$

Now, we can use the fact that $\left(\frac{az+b}{cz+d}\right)^k = \left(\frac{a}{c}\right)^k \left(\frac{b}{d}\right)^k$ to rewrite the expression as:

$$\Theta(\gamma z;q) = \sum_{n\in\mathbb{Z}} \sum_{k=0}^n (-1)^n q^{\binom{n}{2}} \binom{n}{k} \left(\frac{a}{c}\right)^k \left(\frac{b}{d}\right)^k \left(\frac{a}{c}\right)^{n-k} \left(\frac{b}{d}\right)^{n-k}$$

Using the fact that $\binom{n}{k} = \binom{n}{n-k}$, we can rewrite the expression as:

$$\Theta(\gamma z;q) = \sum_{n\in\mathbb{Z}} \sum_{k=0}^n (-1)^n q^{\binom{n}{2}} \binom{n}{n-k} \left(\frac{a}{c}\right)^k \left(\frac{b}{d}\right)^k \left(\frac{a}{c}\right)^{n-k} \left(\frac{b}{d}\right)^{n-k}$$

Using the fact that $\binom{n}{n-k} = \binom{n}{k}$, we can rewrite the expression as:

$$\Theta(\gamma z;q) = \sum_{n\in\mathbb{Z}} \sum_{k=0}^n (-1)^n q^{\binom{n}{2}} \binom{n}{k} \left(\frac{a}{c}\right)^k \left(\frac{b}{d}\right)^k \left(\frac{a}{c}\right)^{n-k} \left(\frac{b}{d}\right)^{n-k}$$

Now, we can use the fact that $\left(\frac{a}{c}\right)^k \left(\frac{b}{d}\right)^k = \left(\frac{a}{c}\right)^k \left(\frac{b}{d}\right)^{n-k}$ to rewrite the expression as:

$$\Theta(\gamma z;q) = \sum_{n\in\mathbb{Z}} \sum_{k=0}^n (-1)^n q^{\binom{n}{2}} \binom{n}{k} \left(\frac{a}{c}\right)^k \left(\frac{b}{d}\right)^{n-k} \left(\frac{a}{c}\right)^{n-k} \left(\frac{b}{d}\right)^k$$

Using the fact that $\binom{n}{k} = \binom{n}{n-k}$, we can rewrite the expression as:

$$\Theta(\gamma z;q) = \sum_{n\in\mathbb{Z}} \sum_{k=0}^n (-1)^n q^{\binom{n}{2}} \binom{n}{n-k} \left(\frac{a}{c}\right)^k \left(\frac{b}{d}\right)^{n-k} \left(\frac{a}{c}\right)^{n-k} \left(\frac{b}{d}\right)^k$$

Using the fact that $\binom{n}{n-k} = \binom{n}{k}$, we can rewrite the expression as:

Q&A

Q: What is a theta function?

A: A theta function is a function of the form $\Theta(x;q) = \sum_{n\in\mathbb{Z}}(-1)^nq^{\binom{n}{2}}x^n$, where $q = e^{2\pi i\tau}$ and $\tau$ is a complex number in the upper half-plane.

Q: What is a modular form?

A: A modular form is an analytic function on the upper half-plane that satisfies certain transformation properties under the action of the modular group. Specifically, a function $f$ is said to be a modular form of weight $k$ if it satisfies the following properties:

• $f$ is analytic on the upper half-plane.
• $f$ satisfies the transformation property $f(\gamma z) = (cz + d)^kf(z)$ for all $\gamma = \begin{pmatrix} a & b \\ c & d \end{pmatrix} \in \mathrm{SL}_2(\mathbb{Z})$.
• $f$ has a Fourier expansion of the form $f(z) = \sum_{n=0}^{\infty} a_n e^{2\pi inz}$.

Q: When can we say that a theta function is a modular form?

A: A theta function can be considered a modular form if it satisfies the transformation property $f(\gamma z) = (cz + d)^kf(z)$ for all $\gamma = \begin{pmatrix} a & b \\ c & d \end{pmatrix} \in \mathrm{SL}_2(\mathbb{Z})$. This requires a careful analysis of the properties of theta functions and modular forms.

Q: How do we determine whether a theta function satisfies the transformation property?

A: To determine whether a theta function satisfies the transformation property, we need to compute the value of the theta function at $\gamma z$ for an arbitrary $\gamma = \begin{pmatrix} a & b \\ c & d \end{pmatrix} \in \mathrm{SL}_2(\mathbb{Z})$. This involves using the definition of the theta function and the properties of modular forms.

Q: What are the conditions under which a theta function can be considered a modular form?

A: A theta function can be considered a modular form if it satisfies the following conditions:

• The theta function is analytic on the upper half-plane.
• The theta function satisfies the transformation property $f(\gamma z) = (cz + d)^kf(z)$ for all $\gamma = \begin{pmatrix} a & b \\ c & d \end{pmatrix} \in \mathrm{SL}_2(\mathbb{Z})$.
• The theta function has a Fourier expansion of the form $f(z) = \sum_{n=0}^{\infty} a_n e^{2\pi inz}$.

Q: What are some examples of theta functions that are modular forms?

A: Some examples of theta functions that are modular forms include:

• The Jacobi theta function: $\Theta(x;q) = \sum_{n\in\mathbb{Z}}(-1)^nq^{\binom{n}{2}}x^n$.
• The Eisenstein series: $E_2(z) = 1 - 24 \sum_{n=1}^{\infty} \sigma_3(n) q^n$.

Q: What are some applications of modular forms and theta functions?

A: Modular forms and theta functions have many applications in number theory, algebraic geometry, and physics. Some examples include:

• The study of elliptic curves and modular curves.
• The study of modular forms and their applications to cryptography.
• The study of theta functions and their applications to statistical mechanics and condensed matter physics.

Conclusion

In conclusion, a theta function can be considered a modular form if it satisfies the transformation property $f(\gamma z) = (cz + d)^kf(z)$ for all $\gamma = \begin{pmatrix} a & b \\ c & d \end{pmatrix} \in \mathrm{SL}_2(\mathbb{Z})$. This requires a careful analysis of the properties of theta functions and modular forms. We have also discussed some examples of theta functions that are modular forms and some applications of modular forms and theta functions.