 Also crucial at the time was the study of the analytic sets, namely the continuous images of Borel sets, or equivalently, the projections of Borel sets. The young Russian mathematician Mikhail Suslin found a mistake in Lebesgue’s 1905 memoir when he realized that the projection of a Borel set is not Borel in general [Suslin 1917]. However, he was able to establish that the analytic sets, too, possess the perfect set property and thus verify CH. By 1923 Nikolai Lusin and Wacław Sierpiński were studying the co-analytic sets, and this was to lead them to a new hierarchy of projective sets, which starts with the analytic sets \((\Sigma^{1}_{1})\), their complements (co-analytic, \(\Pi^{1}_{1}\) sets), the projections of these last (\(\Sigma^{1}_{2}\) sets), their complements (\(\Pi^{1}_{2}\) sets), and so on. During the 1920s much work was done on these new types of sets, mainly by Polish mathematicians around Sierpiński and by the Russian school of Lusin and his students. A crucial result obtained by Sierpiński was that every \(\Sigma^{1}_{2}\) set is the union of \(\aleph_{1}\) Borel sets (the same holds for \(\Sigma^{1}_{1}\) sets), but this kind of traditional research on the topic would stagnate after around 1940 (see Kanamori [1995]).