Linear Tur'an Numbers of Four-Edge Uniform Paths via Incidence Rank

Abstract

Let $P_4^r$ denote the $r$-uniform expansion of the graph path with four edges. We prove that every $n$-vertex linear $r$-uniform $P_4^r$-free hypergraph has at most $(r+1)n/r$ edges, resolving a conjecture of Adak and Verma for every $r \geq 2$. Equality holds precisely for vertex-disjoint unions of Steiner systems $S(2,r,r^2)$. The main ingredient is a sharp incidence-rank inequality. If $N(H)$ is the edge-vertex incidence matrix of a linear $r$-uniform hypergraph whose line graph is a cograph, then $(r+1)\operatorname{rank}_{\mathbb{R}} N(H) \geq r|E(H)|$. Equality holds exactly when every edge-containing component is an $S(2,r,r^2)$. The proof follows the union-join decomposition of cographs. At a join node, the row-difference spaces of the co-components are mutually orthogonal, and the possible rank defect is determined by balanced co-components. Perron-Frobenius theory identifies the smallest balanced pieces as parallel classes of $r$ disjoint $r$-sets, while an orthogonality argument bounds their number by $r+1$. The equality case then reconstructs the Steiner system. The linear Turán bound follows from $\operatorname{rank}_{\mathbb{R}} N(H) \leq |V(H)|$.