An example which discerns porosity and symmetric porosity (Q1189105)

The author investigates relationships among the following four \(\sigma\)- ideals of sets of the reals: \(\sigma\)-porous sets, \(\sigma\)-strongly porous sets, \(\sigma\)-symmetrically porous sets and \(\sigma\)-strongly symmetrically porous sets, and some problems raised by \textit{L. Zajíček} [Real Anal. Exch. 13(1987/88), No. 2, 314-350 (1988; Zbl 0666.26003)] are solved, e.g., it is proved that there exists a \(G_ \delta\) strongly porous set which is not \(\sigma\)-symmetrically porous.