# Relative Interlevel Set Cohomology Categorifies Extended Persistence Diagrams

The extended persistence diagram introduced by Cohen-Steiner, Edelsbrunner, and Harer is an invariant of real-valued continuous functions, which are π½-tame in the sense that all open interlevel sets have degree-wise finite-dimensional cohomology with coefficients in a fixed field π½. We show that relative interlevel set cohomology (RISC), which is based on the MayerβVietoris pyramid by Carlsson, de Silva, and Morozov, categorifies this invariant. More specifically, we define an abelian Frobenius category pres(π₯) of presheaves, which are presentable in a certain sense, such that the RISC h(f) of an π½-tame function f X ββ is an object of pres(π₯), and moreover the extended persistence diagram of f uniquely determines - and is determined by - the corresponding element [h(f)] β K_0 (pres(π₯)) in the Grothendieck group K_0 (pres(π₯)) of the abelian category pres(π₯). As an intermediate step we show that pres(π₯) is the abelianization of the (localized) category of complexes of π½-linear sheaves on β, which are tame in the sense that sheaf cohomology of any open interval is finite-dimensional in each degree. This yields a close link between derived level set persistence by Curry, Kashiwara, and Schapira and the categorification of extended persistence diagrams.

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