Can we assign the Borel hulls in a monotone way?

Márton Elekes, András Máthé

A \emph{hull} of $A\subset [0,1]$ is a set $H$ containing $A$ such that ${\lambda }^{\ast }(H)={\lambda }^{\ast }(A)$. We investigate all four versions of the following problem. Does there exist a monotone (wr…