Tur\'an Problems for Mixed Graphs

We investigate natural Tur\'an problems for mixed graphs, generalizations of graphs where edges can be either directed or undirected. We study a natural \textit{Tur\'an density coefficient} that measures how large a fraction of directed edges an $F$-free mixed graph can have; we establish an analogue of the Erd\H{o}s-Stone-Simonovits theorem and give a variational characterization of the Tur\'an density coefficient of any mixed graph (along with an associated extremal $F$-free family). This characterization enables us to highlight an important divergence between classical extremal numbers and the Tur\'an density coefficient. We show that Tur\'an density coefficients can be irrational, but are always algebraic; for every positive integer $k$, we construct a family of mixed graphs whose Tur\'an density coefficient has algebraic degree $k$.