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arXiv · 2508.11229

On the $PGL_2(q)$-orbits of lines of $PG(3,q)$ and binary quartic forms in characteristic three

Krishna Kaipa·Puspendu Pradhan
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Abstract

We consider the problem of classifying the lines of the projective $3$-space $PG(3,q)$ over a finite field $\mathbb{F}_q$ into orbits of the group $PGL_2(q)$ of linear symmetries of the twisted cubic $C$. The problem has been solved in literature in characteristic different from $3$, and in this work, we solve the problem in characteristic $3$. We reduce this problem to another problem, which is the classification of binary quartic forms into $PGL_2(q)$-orbits. We first solve the latter problem and use to solve the former problem. We also obtain the point-line and the line-plane incidence structures of the point, line, and plane orbits.

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Krishna Kaipa, Puspendu Pradhan. 2025-08-15. On the $PGL_2(q)$-orbits of lines of $PG(3,q)$ and binary quartic forms in characteristic three. https://arxiv.org/abs/2508.11229

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