Pure subrings of Du Bois singularities are Du Bois singularities

Abstract Let $R \to S$ be a cyclically pure map of Noetherian $\mathbb {Q}$ -algebras. In this paper, we show that if S has Du Bois singularities, then R has Du Bois singularities. Our result is new even when $R \to S$ is faithfully flat. Our proof also yields interesting results in prime characteristic and in mixed characteristic. As a consequence, we show that if $R \to S$ is a cyclically pure map of rings essentially of finite type over the complex numbers $\mathbb {C}$ , S has log canonical type singularities, and $K_R$ is Cartier, then R has log canonical singularities. Along the way, we prove a version of the key injectivity theorem of Kovács and Schwede for Noetherian schemes of equal characteristic zero that have isolated non-Du Bois points. Throughout the paper, we use the characterization of the complex $\underline {\Omega }^0_X$ and of Du Bois singularities in terms of sheafification with respect to Grothendieck topologies.