Four Special Directions in AG(2,13): The 52-Point Obstruction and the Sharp Minimum

Source abstract

We prove that no $52$-point subset of the affine plane $\mathbb F_{13}^{2}$ has exactly four special directions, where a direction is special when its thirteen parallel affine lines do not all meet the set in the same number of points. A universal incidence identity reduces the four exceptional line-count functions to a polynomial identity over $\mathbb F_{13}$. Linear independence of the associated binary forms of degree at least three then forces those functions to have degree at most two. Classification of the resulting constant, linear, and quadratic profiles leaves a quadratic-character congruence with no solution. The incidence and polynomial argument supplies the quantifier over all $52$-point subsets and all four-direction sets; the remaining classification of quadratic value tables is finite and exact. Together with Ghidelli's lower bound and the $65$-point construction of Kiss and Somlai, this determines the minimum size of a subset of $\mathbb F_{13}^{2}$ with exactly four special directions: it is $65$. The AI-assisted workflow used OpenAI GPT-5.6 Sol, Anthropic Claude Fable 5, Grok 4.6, and OpenAI GPT-6 Astra, together with Codex-controlled Danus.