Complex sentence: He then wrote a doctoral dissertation on characteristic classes under Friedrich Hirzebruch at Bonn, graduating at 21, and later collaborated with Hirzebruch in work on Hilbert modular surfaces.
Split into simpler sentences: He then wrote a doctoral dissertation on characteristic classes under Friedrich Hirzebruch at Bonn, receiving his PhD at 21. He has collaborated with Hirzebruch in work on Hilbert modular surfaces.