PRiME 2015: Purdue Research in Mathematics Experience

[<< PRiME 2014] | [PRiME Main Page] | [PRiME 2016 >>]

      

The Department of Mathematics at Purdue University offered an 8-week residential program to conduct research in pure mathematics from June 15, 2015 through August 7, 2015. The program was sponsored by the National Science Foundation (NSF) as well as generous gifts from Ruth and Joel Spira (BS '48, Physics) and Andris "Andy" Zoltners (MS '69, Mathematics).


Background and Project Description

Let \(X\) be a compact, connected Riemann surface. It is well-known that \(X\) is an algebraic variety, that is, \(X \simeq \left \{ P \in \mathbb P^n(\mathbb C) \, \bigl| \, F_1(P) = F_2(P) = \cdots = F_m(P) = 0 \right \}\) in terms of a collection of homogeneous polynomials \(F_i\) over \(\mathbb C\) in \((n+1)\) variables \(x_j\). Denote \(\mathcal O_X\) as the ring of regular functions on \(X\), that is, ``polynomials'' \(f, g: X \to \mathbb P^1(\mathbb C)\); and denote \(\mathcal K_X\) as its quotient field, that is, rational functions \(f/g: X \to \mathbb P^1(\mathbb C)\). For example, if \(X \simeq \mathbb P^1(\mathbb C) = \mathbb C \cup \{ \infty \}\), then \(\mathcal O_X \simeq \mathbb C[z]\) consists of polynomials in one variable, while \(\mathcal K_X \simeq \mathbb C(z)\) consists of rational functions in one variable. In particular, any rational map \(\beta: X \to \mathbb P^1(\mathbb C)\) induces a map \(\beta^\ast: \mathbb C(z) \to \mathcal K_X\) which sends \(J \mapsto J \circ \beta\). The degree of such a map is the size of the group \(G = \text{Gal} \bigl( \mathcal K_X / \beta^\ast \, \mathbb C(z) \bigr)\).

For each \(P \in X\), let \(\mathcal O_P\) be the localization of \(\mathcal O_X\) at the kernel of the evaluation map \(\mathcal O_X \to \mathbb C\) defined by \(f \mapsto f(P)\). Let \(\mathfrak m_P\) denote the maximal ideal of \(\mathcal O_P\); we view this as the collection of rational maps \(\beta \in \mathcal K_X\) which vanish at \(P\). As shown by Weil and Belyĭ, the Riemann surface \(X\) can be defined in terms of homogeneous polynomials \(F_i\) over an algebraic closure \(\overline{\mathbb Q}\) if and only if there exists a rational function \(\beta: X \to \mathbb P^1(\mathbb C)\) such that \(\beta: \left \{ P \in X \, \bigl| \, \beta - \beta(P) \in {\mathfrak m_P}^2 \right \} \to \bigl \{ 0, \, 1, \, \infty \}\). The difference \(\beta - \beta(P) \in \mathfrak m_P\) for any \(P \in X\) because the function vanishes at \(P\); the condition \(\beta - \beta(P) \in {\mathfrak m_P}^2\) means the derivative of the function vanishes as well. A rational function as above where these critical values are at most \(0\), \(1\), and \(\infty\) is called a Belyĭ map.


2015 Research Questions

Following Grothendieck, we associate a bipartite graph \(\Delta_\beta\) to a Belyĭ map \(\beta: X \to \mathbb P^1(\mathbb C)\) by denoting the ``black'' vertices as \(B = \beta^{-1}(0)\), ``white'' vertices as \(W = \beta^{-1}(1)\), midpoints of the faces as \(F = \beta^{-1}(\infty)\), and edges as \(E = \beta^{-1}\bigl([0,1] \bigr)\). This is a loopless, connected, bipartite graph, called a Dessin d'Enfant, which can be embedded on \(X\) without crossings. The group \(G = \text{Gal} \bigl( \mathcal K_X / \beta^\ast \, \mathbb C(z) \bigr)\) permutes the solutions \(P\) to \(\beta(P) = z\), and hence acts on the dessin \(\Delta_\beta\). The hope is that in studying graphs \(\Delta_\beta\) one can better understand quotients \(G\) of the absolute Galois group \(\text{Gal} \bigl( \overline{\mathbb Q}/\mathbb Q \bigr)\).

There were two main projects.


Group #1: Examples of Belyĭ Maps for Elliptic Curves

Abstract. A Belyĭ map \(\beta: \mathbb P^1(\mathbb C) \to \mathbb P^1(\mathbb C)\) is a rational function with at most three critical values; we may assume these values are \(\{ 0, \, 1, \, \infty \}\). Replacing \(\mathbb P^1\) with an elliptic curve \(E: \ y^2 = x^3 + A \, x + B\), there is a similar definition of a Belyĭ map \(\beta: E(\mathbb C) \to \mathbb P^1(\mathbb C)\).

The first aspect of this project seeks to determine infinite families of Belyĭ maps for elliptic curves. We have shown that given any elliptic curve \(E\) there exist infinitely many Belyĭ maps of degree 2; they are in the form \(\beta(x,y) = (a \, x + b) / (c \, x + d)\). We have also shown that any elliptic curve has at least one Belyĭ map of degree 3 with critical values \(\{ 0, \, 1, \, \infty \}\). After placing the curve in Hessian normal form \(y^2 + a_1 \, x \, y + a_3 \, y = x^3\), the Belyĭ map is in the form $$\beta(x,y) = \dfrac {\bigl( 2 \, a_1^3 - 27 \, a_3 + 2 \, \sqrt{a_1^6 - 27 \, a_1^3 \, a_3} \bigr) \, y - 27 \, a_3^2}{\bigl( 2 \, a_1^3 - 27 \, a_3 - 2 \, \sqrt{a_1^6 - 27 \, a_1^3 \, a_3} \bigr) \, y - 27 \, a_3^2} \qquad \text{for \(a_1 \neq 0\).}$$

The second aspect of this project seeks to determine of Belyĭ maps for elliptic curves corresponding to the complete bipartite graphs. A 2009 paper by Coste, Jones, Streit, and Wolfart claims that the Utility graph \(K_{3,3}\) may be realized as a Dessin d'Enfant coming from the Belyĭ map \(\beta: (x_1 : x_2 : x_0) \mapsto (x_1 / x_0)^3\) on the elliptic curve \(E: x_1^3 + x_2^3 + x_0^3 = 0\). It is known that the complete bipartite graphs which can be embedded on the torus (and not on the sphere) are \(K_{3,3}\), \(K_{3,4}\), \(K_{3,5}\), \(K_{3,6}\), and \(K_{4,4}\). Riemann's Existence Theorem asserts that each of these can be visualized as a Dessin d'Enfant. In this project, we seek to find explicit corresponding Belyĭ maps.


Group #2: Visualizing Dessins d'Enfants on the Torus

Abstract. A Belyĭ map \(\beta: \mathbb P^1(\mathbb C) \to \mathbb P^1(\mathbb C)\) is a rational function with at most three critical values; we may assume these values are \(\{ 0, \, 1, \, \infty \}\). A Dessin d'Enfant is a planar bipartite graph obtained by considering the preimage of a path between two of these critical values, usually taken to be the line segment from 0 to 1. Such graphs can be drawn on the sphere by composing with stereographic projection: \(\beta^{-1} \bigl( [0,1] \bigr) \subseteq \mathbb P^1(\mathbb C) \simeq S^2(\mathbb R)\). Replacing \(\mathbb P^1\) with an elliptic curve \(E\), there is a similar definition of a Belyĭ map \(\beta: E(\mathbb C) \to \mathbb P^1(\mathbb C)\). The corresponding Dessin d'Enfant can be drawn on the torus by composing with an elliptic logarithm: \(\beta^{-1} \bigl( [0,1] \bigr) \subseteq E(\mathbb C) \simeq \mathbb T^2(\mathbb R)\).

In this project, we use the open source Sage to write code which takes an elliptic curve \(E\) and a Belyĭ map \(\beta\) to return a Dessin d'Enfant on the torus -- both in two and three dimensions. Following a 2013 paper by Cremona and Thongjunthug we make the elliptic logarithm \(E(\mathbb C) \simeq \mathbb C / \Lambda\) explicit using a modification of the arithmetic-geometric mean, then compose with a canonical one-to-one correspondences \(\mathbb C / \Lambda \simeq \mathbb T^2(\mathbb R)\). Using this, we focus on several examples of Belyĭ maps which appear on Elkies' Harvard web page entitled ``Elliptic Curves in Nature.''


[<< PRiME 2014] | [PRiME Main Page] | [PRiME 2016 >>]