Characteristic functions and products of derivatives
The following Theorem is proven: For a subset S of the reals R the following are mutually equivalent: (1) the characteristic function of S is a product of finitely many derivatives; (2) S is ambiguous and \(T=R\setminus S\) is porous; (3) there exist derivatives f and g such that \(f=g=1\) on S and \(fg=0\) on T.
scientific article; zbMATH DE number 4210453