Spectre operator, achievement sets and sets of P-sums in a hyperspace of compact sets

Piotr Nowakowski, Franciszek Prus-Wiśniowski, Filip Turoboś

Let $(X,+,d)$ be an Abelian metric group and $A\subset X$. We investigate the spectre of a set $A$, defined as the set of all elements $z\in X$ such that for every $x\in A$ either…