# The PIMS Postdoctoral Fellow Seminar: Zafer Selcuk Aygin (Online)

## Topic

Relations between \(\triangle+\cdots + \triangle + 3\triangle+\cdots + 3\triangle\) and \(\square+\cdots + \square + 3\square+\cdots + 3\square\)

## Details

For non-negative integers \(a,b\) and \(n\), let $$ \begin{align*} t(a,b;n)=& \# \left\{ (x_1,\ldots,x_a,y_1,\ldots, y_b) \in \mathbb{Z}^{a+b} ~ \mid ~n= \frac{x_1(x_1-1)}{2} + \cdots + \frac{x_a(x_a-1)}{2} \right. \\ & \left. + 3\frac{y_1(y_1-1)}{2}+ \cdots + 3\frac{y_b(y_b-1)}{2} \right\} \end{align*} $$ And $$ \begin{align*} N(a,b;n)= & \# \{ (x_1,\ldots,x_a,y_1,\ldots, y_b) \in \mathbb{Z}^{a+b} ~\mid~ n=x_1^2 + \cdots + x_a^2 \\ & + 3y_1^2+ \cdots + 3y_b^2 \}. \end{align*} $$ By works of Bateman and Knopp & Adiga, Cooper and Han, it is known that for \((a,b) = (1,0), (2,0), (3,0), (4,0), (5,0), (6,0), (7,0)\), and \((3,1)\) we have $$ \begin{align*} \frac{t(a,b;n)}{N(a,b;8n+a+3b)}=\frac{2}{2^{a+b-2} + 2^{(a+b-2)/2} \cos(\pi (a+3b)/4) +1} \end{align*} $$ for all \(n \in \mathbb{N}\). Moreover, for \((a,b) = (8,0)\) and \((2,2)\), Baruah, Cooper and Hirschhorn has proven that $$ \begin{align*} \frac{t(a,b;n)}{N(a,b;8n+a+3b)-N(a,b;(8n+a+3b)/4)} = \frac{2}{2^{a+b-2} + 2^{(a+b-2)/2}} \end{align*} $$ for all \(n \in \mathbb{N}\). Variations of such identities were observed by many mathematicians including Baruah, Bateman, Cooper, Dastovski, Hirschhorn, Knopp, Sun and Williams. However, it seems that for bigger values of \(a\) and \(b\) these ratios start to fluctuate when \(n\) varies. In this work we investigate the limiting cases of similar ratios when \(a+b\) is an even integer greater than 4, \(a \geq 2\) and \(b \geq 0\). We show that previously known examples are special cases of an asymptotic relation between \(N(a,b;n)\) and \(t(a,b;n)\). We achieve our results by finding certain modular identities which relates generating functions of \(t(a,b;n)\) to \(N(a,b;n)\). This is a joint work with Amir Akbary (University of Lethbridge).

This event is part of the Emergent Research: The PIMS Postdoctoral Fellow Colloquium Series.

This event took place via zoom and a recording is available on mathtube.org. To learn about other events in this series and to receive connection details, please register for the event mailing list.

## Additional Information

Zafer Selcuk Aygin

Bio: Zafer Selcuk Aygin completed his Ph. D. in mathematics at Carleton University in 2016. He was a research fellow at Nanyang Technological University, Singapore until 2018 and is currently holding a PIMS Post-Doctoral Fellowship at the University of Lethbridge and the University of Calgary on a joint appointment, supported by the Pacific Institute for Mathematical Sciences

**Scientific, Seminar**

**November 25, 2020**

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