Weak Spectral Synthesis
C. Robert Warner · Proceedings of the American Mathematical Society · 1987
As an approach to the union problem for sets of spectral synthesis ($S$-sets), the class of weak $S$-sets is introduced. This class contains all finite unions of $S$-sets, and it has many properties analogous to those of Calderón sets. It is closed under the operation of forming finite unions, but, in contrast to Calderón sets, it is not closed under countable unions.