A photo of the German mathematician Georg Cantor, mentioned in the Did You Know? section. Cantor formulated set theory, a fundamental theory in mathematics. Photo: https://en.wikipedia.org/wiki/georg_cantor

Right: A representation of the first few ordinal numbers, mentioned in the Did You Know? section. each turn of the spiral represents one power of ω. Photo: https://en.wikipedia.org/wiki/ordinal_number