Perfectoid Pure and Signature Pairs

Abstract: We extend perfectoid purity and related invariants to divisor pairs in mixed characteristic. The resulting purity condition agrees with $F$-purity of pairs in characteristic $p$ as defined by Hara--Watanabe. For normal (respectively, demi-normal) log $\mathbb{Q}$-Gorenstein pairs, we show that perfectoid pure implies log canonical (respectively, semi-log canonical). We similarly establish adjunction for perfectoid pure pairs and inversion of adjunction for principal pairs in complete intersections along a Cartier divisor. We also define perfectoid pure thresholds and prove localization and base change results, including a formula for local extensions with regular closed fiber. We establish upper bounds in terms of analytic spread, dimension, and log canonical thresholds. Finally, we also introduce perfectoid signatures for divisor pairs in mixed characteristic whose positivity characterizes the BCM-regularity of log $\mathbb{Q}$-Gorenstein pairs. We prove a transformation rule under finite extensions and show continuity and convexity for scalings of a Cartier divisor.