who can be regarded as the last mathematician to be active over the entire field of mathematics. Within pure mathematics, he is seen as one of the main founders of topology. In applied mathematics, he is remembered for his theoretical work on the qualitative aspects of celestial mechanics, which was probably the most important work in that area since Laplace and Lagrange. Connecting and motivating both was his qualitative theory of differential equations. He was also known for a stream of popular books for the aspiring young mathematician.