The Variance of the Sum of Two Squares over Intervals in $\mathbb{F}_q [T]$: I

Abstract: For $B \in \mathbb{F}q [T]$ of degree $2n \geq 2$, consider the number of ways of writing $B=E^2 + \gamma F^2$, where $\gamma \in \mathbb{F}_q^*$ is fixed, and $E,F \in \mathbb{F}_q [T]$ with $\mathrm{deg} \hspace{0.25em} E = n$ and $\mathrm{deg} \hspace{0.25em} F = m < n$. We denote this by $S{\gamma ; m} (B)$. We obtain an exact formula for the variance of $S_{\gamma ; m} (B)$ over intervals in $\mathbb{F}_q [T]$. We use the method of additive characters and Hankel matrices that the author previously used for the variance and correlations of the divisor function. In Section 2, we give a short overview of our approach; and we briefly discuss the possible extension of our result to the number of ways of writing $B=E^2 + T F^2$.