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arXiv · 2203.16709

Class Numbers and Pell's Equation $x^2 + 105y^2 = z^2$

Thomas Jaklitsch·Thomas C. Martinez·Steven J. Miller·Sagnik Mukherjee
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Abstract

Two well-studied Diophantine equations are those of Pythagorean triples and elliptic curves, for the first we have a parametrization through rational points on the unit circle, and for the second we have a structure theorem for the group of rational solutions. Recently, Yekutieli discussed a connection between these two problems, and described the group structure of Pythagorean triples and the number of triples for a given hypotenuse. In arXiv:2112.03663 we generalized these methods and results to Pell's equation. We find a similar group structure and count on the number of solutions for a given $z$ to $x^2 + Dy^2 = z^2$ when $D$ is 1 or 2 modulo 4 and the class group of $\mathbb{Q}[\sqrt{-D}]$ is a free $\mathbb{Z}_2$ module, which always happens if the class number is at most 2. In this paper, we discuss the main results of arXiv:2112.03663 using some concrete examples in the case of $D=105$.

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Thomas Jaklitsch, Thomas C. Martinez, Steven J. Miller, Sagnik Mukherjee. 2022-03-30. Class Numbers and Pell's Equation $x^2 + 105y^2 = z^2$. https://arxiv.org/abs/2203.16709

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