$L_2$ Turán Problems for Small Tournaments and Stability

Source abstract

We investigate the $L_2$ Turán problems for various small directed graphs, specifically focusing on self-converse tournaments and stability versions. First, we determine the exact maximum $L_2$ norm squared of the out-degree sequence for digraphs avoiding the transitive tournament $TT_4$ and the strongly connected tournament $R_4$, answering open questions from recent paper. We prove that the complete directed 3-partite Turán graph $T_3(m)$ exactly maximizes the $L_2$ norm squared for $TT_4$-free digraphs. For $R_4$-free digraphs, the maximum is achieved by $T_3(m)$ except when $m \equiv 1 \pmod 3$, where peeling off a terminal sink vertex to form $T_3(m-1) \to v$ strictly increases the objective. We complement these results with exact values and a conjecture for the regular tournament $Reg_5$. Furthermore, we prove a stability version for $\vec{C}_3$-free digraphs: any sequence of digraphs asymptotically achieving the maximum $L_2$ density must have an edit distance of $O(δ^{1/2})m^2$ to the extremal ordered digon-chain $\vec{F}_{m,2}$.