His topological work concentrated mainly on the fundamental group; many of the ideas he pioneered were innovations in topology as well as algebraic geometry and have developed independently in the two fields since then.
Topological kanyang trabaho puro pangunahin sa mga pangunahing grupo; marami sa mga ideya niya pioneered ay mga makabagong-likha sa topology pati na rin ang algebraic geometry at may binuo nang malaya sa dalawang mga patlang dahil pagkatapos.