Local Poincar\'e constants and mean oscillation functionals for $BV$ functions

Abstract

We introduce the concept of local Poincar\'e constant of a $BV$ function as a tool to understand the relation between its mean oscillation and its total variation at small scales. This enables us to study a variant of the BMO-type seminorms on $\varepsilon$-size cubes introduced by Ambrosio, Bourgain, Brezis, and Figalli. More precisely, we relax the size constraint by considering a family of functionals that allow cubes of sidelength smaller than or equal to $\varepsilon$. These new functionals converge, as $\varepsilon$ tends to zero, to a local functional defined on $BV$, which can be represented by integration in terms of the local Poincar\'e constant and the total variation. This contrasts with the original functionals, whose limit is defined on $SBV$ and may not exist for functions with a non-trivial Cantor part. Moreover, we characterize the local Poincar\'e constant of a function with a cell-formula given by the maximum mean oscillation of its $BV$ blow-ups. As a corollary of this characterization, we show that the new limit functional extends the original one to all $BV$ functions. Finally, we discuss rigidity properties and other challenging questions relating the local Poincar\'e constant of a function to its fine properties.