# Elliptic functions, Part I

Elliptic functions were discovered in the ${19^{th}}$ century, but their first appearance in hidden guise goes back to Fagano and Euler, who proved “addition theorems” for elliptic integrals which amount to addition theorems for elliptic functions. Elliptic functions were the bread and butter of many generations of mathematicians; their study gave birth to the theory of Riemann surfaces and, eventually, to modern complex geometry.

An elliptic function ${f}$ is a meromorphic function on ${\mathbb{C}}$ which has two ${\mathbb{R}}$-linearly independent periods ${\omega_1}$ and ${\omega_2}$. Thus ${f}$ is invariant under the action of the lattice group ${\Omega = \omega_1{\mathbb Z}\oplus \omega_2{\mathbb Z}}$. (Of course, there are many possible choices of ${\omega_1}$ and ${\omega_2}$.) This means that we can factor ${f}$ through the projection ${\mathbb{C}\rightarrow \mathbb{C}/\Omega}$. With the complex structure inherited from ${\mathbb{C}}$, the topological space ${E_\Omega = \mathbb{C}/\Omega}$ is a compact Riemann surface which has the shape of a torus.

We can identify ${E_\Omega}$ with the points of a suitably chosen parallelogram. The parallelogram having ${0}$, ${\omega_1}$ and ${\omega_2}$ as vertices is called a fundamental parallelogram. We include only a specified half of its boundary (for example, only the edges ${0\omega_1}$ and ${0\omega_2}$) so as to make sure that no two points are congruent ${\mod \Omega}$. Of course, ${E_\Omega}$ is simply obtained by “pasting” opposite sides of this parallelogram together. We will call any fundamental parallelogram ${E}$. (Thus, as a topological space, ${E_\Omega}$ is a quotient of ${E}$, obtained by pasting opposite sides together.) There are many other choice for $E_\Omega$; as many as there are ${\mathbb{Z}}$-bases for ${\Gamma}$.

So, all that we have merely noted so far is the basic fact that there exists a natural identification of the family of all ${\Omega}$-invariant meromorphic functions on ${\mathbb{C}}$ with the family of all meromorphic functions on ${\mathbb{C}/\Omega}$.

So now we want to build elliptic functions. The way to build a function invariant under the action of some group is to average out this action. For example, suppose ${G}$ is a finite group acting on some set ${S}$ and that we are given a function ${f:S\rightarrow \mathbb{C}}$. We can build the function ${F:S \rightarrow \mathbb{C}}$ by ${F(s)=\sum_{g \in G}f(g \cdot s)}$, which is invariant under ${G}$. Of course, if ${G}$ is infinite, we may hope to replace the finite sum by an appropriately converging series.

So Weierstrass’s idea was just that. Let ${G=\Omega}$, and let ${f(z)=\frac{1}{z^{3}}}$. The series

$\displaystyle f(z)=\sum_{\omega \in \Omega}\frac{1}{(z-\omega)^3}$

is easily seen to converge absolutely and uniformly on every compact set not containing a point of ${\Omega}$. Since each summand is a meromorphic function of ${z}$, so is ${f(z)}$.

It is easy to see from the series expansion that the function ${f}$ has a triple pole at every lattice point with zero residue. Moreover, ${f(z)}$ is odd, since ${-\Omega = \Omega}$ and ${z^3}$ is odd. Thus we have produced a non-trivial elliptic function of order ${3}$ (the order, or the degree, of an elliptic function ${f}$ is the number of its poles inside any period parallelogram; or, if you prefer, it is the degree of ${f}$ considered as a ramified covering ${E_\Omega \rightarrow \mathbb{CP}^1}$).

Now for every ${z \notin \Gamma}$, the function ${f(u)-1/u^3}$ can be integrated from ${0}$ to ${z}$ along a path not passing through any point of ${\Gamma}$. Since the residue of ${f}$ at each pole is ${0}$, the value of the integral is independent of path. By the uniform convergence of the series defining ${f}$ along the integration path, we obtain a new function of ${z}$,

$\displaystyle P(z)=\int_0^z (f(u)-1/u^3)du = \frac{-1}{2}\sum_{\omega \in \Omega^*} \left(\frac{1}{(z-\omega)^2}-\frac{1}{\omega^2}\right),$

which is meromorphic, and has a double pole with zero residue at every point of ${\Omega^*=\Omega - \{0\}}$. The function ${\wp(z)=-2P(z)+2u^{-2}}$ is the Weierstrass ${\wp}$-function associated to ${\Omega}$ (we may sometimes write it ${\wp_\Omega}$ to emphasize the dependence of ${\wp}$ on ${\Omega}$). It is given by the series

$\displaystyle \wp(z)=\frac{1}{z^2}+\sum_{\omega \in \Omega^*} \left(\frac{1}{(z-\omega)^2}-\frac{1}{\omega^2}\right),$

which also converges absolutely and uniformly on every compact set disjoint from ${\Omega}$. Notice that to integrate from ${0}$, we had to remove the pole at ${0}$, integrate, and then put the pole back. It is not immediately obvious that ${\wp}$ should be elliptic. However, since ${f}$ is odd and elliptic, we have, for example, ${f(\omega_1(\frac{1}{2}+t))=f(\omega_1(\frac{-1}{2}+t))=-f(\omega_1(\frac{1}{2}-t))}$. This shows that integrating ${f}$ along the side ${\omega_1}$ of the fundamental period parallelogram gives ${0}$; by generalizing this observation, we can see that ${P(z)}$ is elliptic.

It would be criminal to continue without mentioning that we have only been generalizing the theory of trigonometric functions (and, as we shall see, of their associated curves, the conics). Recall that Euler gave us the product formula for ${\sin z}$:

$\displaystyle \frac{\sin \pi z}{\pi z}=\prod_{n=1}^\infty \left(1-\frac{z^2}{n^2\pi^2}\right).$

Taking the logarithmic derivative, we obtain the “partial fraction” expansion

$\displaystyle \pi \cot \pi z = \frac{1}{z} + \sum_{n=1}^\infty \frac{-2z}{n^2\pi^2}\frac{n^2\pi^2}{n^2\pi^2-z^2}=\frac{1}{z}+\sum_{n=1}^\infty \frac{1}{\pi n-z}-\frac{1}{\pi n+z}.$

(Euler used this formula to give the value of ${\zeta(2n)}$, by expanding further each term in this formula, and comparing the resulting series with the Taylor series for ${\pi \cot \pi z}$.) But this function is not quite yet analogous to the ${\wp}$ function, because it’s an odd function. Applying ${-\frac{d}{dz}}$ yields

$\displaystyle \pi^2 \csc^\pi z = \frac{1}{z^2}+\sum_{n=1}^\infty \frac{1}{(z+\pi n)^2}+\frac{1}{(z-\pi n)^2}$

which really is analogous to the ${\wp}$ function. So we see that the ${\wp}$ function degenerates to ${\csc^2}$ as one of its periods becomes ${0}$.

Now let’s make some general observations about any elliptic function ${f}$. First, note that ${f}$ must have finitely many poles in any period parallelogram, since the closure of the period parallelogram is compact, and ${f}$ is meromorphic. Second, note that the sum of the residues of ${f}$ at its poles in a period parallelogram is ${0}$. Indeed, integrating around the boundary and using the periodicity of ${f}$, we see that integrals along opposite sides cancel each other. (We have to avoid poles on the boundary if there are some. By the periodicity of ${f}$, the poles on the boundary come in pairs with equal residue, and by going around them in small semi-circles in such a way that the contributions of the residues cancel each other out, we save the situation. So we’re integrating on a jigsaw puzzle piece with which we can tile the plane, basically.) This observation may remind you of the theorem which states that the sum of the residues of a meromorphic differential on ${\mathbb{CP}^1}$ is ${0}$. This fact holds on all compact Riemann surfaces.

As a consequence of the fact that the residues sum to ${0}$, we see that an elliptic function cannot have a single simple pole. This is almost true also on ${\mathbb{CP}^1}$. For example, ${f(z)=1/z}$ has a single simple pole. The sum of its residues is not ${0}$, but the differential ${\frac{dz}{z}}$, however, has two poles with residues ${1}$ and ${-1}$; indeed, let ${w=\frac{1}{z}}$; then ${\frac{dz}{z}=-\frac{dw}{w}}$, so that ${\frac{dz}{z}}$ also has a pole at ${w=0}$, with residue ${-1}$. Residues are really a property of differentials and not of functions.

Moreover, a non-constant elliptic function ${f}$ must have at least one pole inside any fundamental parallelogram. Indeed, if ${f}$ is analytic (and hence continuous) on the closure ${\overline{E}}$ of a fundamental parallelogram ${E}$, the image ${f(\overline{E})}$ is compact, since ${\overline{E}}$ is compact; but since ${f(\overline{E})=f(\mathbb{C})}$, Liouville’s theorem implies that ${f}$ is constant.

This observation, while very simple, is the basic tool in proofs of relations among elliptic functions.

Let’s expand the series for ${\wp}$ a bit further. We have

$\displaystyle \frac{1}{(z-\omega)^2}-\frac{1}{\omega^2} = \frac{1}{\omega^2} \left(\frac{1}{(z/\omega-1)^2}-1 \right)=\sum_{n=1}^\infty (n+1)\omega^{-n-2}z^{n}$

Hence

$\displaystyle \wp(z)=\frac{1}{z^2}+\sum_{\omega \in \Omega^*}\left(\frac{1}{(z-\omega)^2}-\frac{1}{\omega^2}\right)$

$\displaystyle = \frac{1}{z^2}+\sum_{n=1}^\infty (n+1)e_{n} z^{n} = \frac{1}{z^2}+2e_1z+3e_2z^2+\dots$

where ${e_{n} = \sum_{\omega \in \Omega^*}\omega^{-n-2}}$. Notice that ${e_n=0}$ for ${n}$ odd, since ${-\Omega = \Omega}$ (or alternatively, since ${\wp}$ is an even function). Thus we have

$\displaystyle \wp(z)=\frac{1}{z^2} + 3e_2z^2 + 5e_4z^4+\dots$

As functions of ${\Gamma}$, the values ${e_4, e_6, \dots, }$ are very interesting in their own right (they’re the fundamental examples of modular forms); I will talk about them in a later post.

So now that we have the Laurent expansion for ${\wp (z)}$ around ${0}$, we can hope to discover a relationship between ${\wp(z)}$ and ${\wp'(z)}$. We have

$\displaystyle \wp'(z)=\frac{-2}{z^3}+6e_2z+20e_4z^3+\dots$

and hence the function ${\wp'(z)^2-4\wp(z)^3}$ has a pole of order ${< 6}$ at ${0}$, since the terms in ${z^{-6}}$ cancel out. We can write explicitly:

$\displaystyle \wp'(z)^2-4\wp(z)^3 = \left(\frac{-24e_2}{z^2}-{80e_4}+\dots \right)-4\left(\frac{9e_2}{z^2}+15e_4+\dots\right)$

$\displaystyle =\frac{-60e_2}{z^2}-140e_4+\dots$

where each occurence of “${\dots}$” represents some analytic function which vanishes at ${0}$. Hence, by adding ${60e_2\wp(z)}$ and ${140e_4}$ to this series, we cancel the ${z^{-2}}$ term and the constant term, and we see that the function ${\wp'(z)^2-4\wp(z)^3+60e_2\wp(z)+140e_4}$ is analytic at ${0}$ and vanishes there. Moreover, it is an elliptic function with respect to ${\Omega}$, since the same is true of ${\wp'}$ and ${\wp}$. Also, it can only have poles at the points of ${\Omega}$, since the same is true of ${\wp'}$ and ${\wp}$. But it has no pole at ${0}$; hence it has no pole at all. Hence it must be constantly equal to ${0}$. So we have proved that the ${\wp}$ function satisfies the second-order non-linear differential equation

$\displaystyle \wp'(z)^2=4\wp(z)^3-g_2\wp(z)-g_3$

in terms of the constants (depending on ${\Omega}$)

$\displaystyle g_2=60\sum_{\omega \in \Omega^*}\frac{1}{\omega^4},$

$\displaystyle g_3=140\sum_{\omega \in \Omega^*}\frac{1}{\omega^6}.$

We will discuss the great significance of this differential equation in a future post.