ANUPAM SINGH
Articles written in Proceedings – Mathematical Sciences
Volume 130 All articles Published: 1 May 2020 Article ID 0031 Research Article
Computing $n$-th roots in $\rm{SL_{2}}$ and Fibonacci polynomials
Let $k$ be a field of characteristic $\neq$2. In this paper, we study squares, cubesand their products in split and anisotropic groups of type $A_{1}$. In the split case,we show that computing $n$-th roots is equivalent to finding solutions of certain polynomial equations in at most two variables over the base field $k$. The description of these polynomials involves generalised Fibonacci polynomials. Using this we obtain asymptotic proportions of $n$-th powers, and conjugacy classes which are $n$-th powers, in $SL_{2}(\mathbb{F}_{q})$ when $n$ is a prime or $n = 4$. We also extend the already known Waring type result for $SL_{2}(\mathbb{F}_{q})$, that every element of $SL_{2}(\mathbb{F}_{q})$ is a product of two squares, to $SL_{2}(k)$ for an arbitrary $k$. For anisotropic groups of type $A_{1}$, namely $SL_{1}(Q)$ where $Q$ is a quaternion division algebra, we prove that when 2 is a square in $k$, every element of $SL_{1}(Q)$ is a product of two squares if and only if $−1$ is a square in $SL_{1}(Q)$.
Volume 130, 2020
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