scratch

§ Fundamental Group Functor Does Not Preserve Epis

created 2022-04-28 · last edited 2023-04-02
  • Epis in the category of topological spaces are continuous functions that have dense image.
  • Take a circle S1S^1S1 and pinch it in the middle to get S1∨S1S^1 \lor S^1S1∨S1. this map is an epi: f:S1→S1∨S1f: S^1 \to S^1 \lor S^1f:S1→S1∨S1.
  • See that this does not induce an epi π(Z)→π(Z)⋆π1(Z)\pi(Z) \to \pi_(Z) \star \pi_1(Z)π(Z)→π(​Z)⋆π1​(Z).
  • Maybe even more simply, the map f:[0,1]→S1f: [0, 1] \to S^1f:[0,1]→S1 is an epi
  • Thus, fundamental group functor does not preserve epis.
❦
Newer ৪ Blog ৪ Older