i'm really stuck in calculating the fundamental group (pi1 ) of this surface... can someone please help me?.... i'm trying to use the van Kampen theorem but i don't know how to take the open sets here to work

enter image description here

  • $\begingroup$ If I interprete your drawing well, this is homeomorphic to a torus, so no need to use Van Kampen $\endgroup$
    – ArtW
    Commented Apr 20, 2018 at 14:00
  • $\begingroup$ torus? i can't understand how? $\endgroup$
    – user297564
    Commented Apr 20, 2018 at 14:02
  • 1
    $\begingroup$ the knotted tube in the middel is just homeomorphic to a straight tube $\endgroup$
    – ArtW
    Commented Apr 20, 2018 at 14:04
  • $\begingroup$ @ArtW : Thank you. $\endgroup$
    – HK Lee
    Commented Apr 20, 2018 at 14:06
  • $\begingroup$ but it is clinging from both sides to the sphare... sorry i can't understand how is it isomorphic to a straght line, if i dont consider the both sides it's tru but now... $\endgroup$
    – user297564
    Commented Apr 20, 2018 at 14:07

1 Answer 1


As ArtW pointed out in the comments, the surface seems homeomorphic to a torus. This can be argued as follows:

  • The knotted tunnel in the middle of the sphere never self-intersects or interacts with the sphere, so it is homeomorphic to a straight tunnel passing through the sphere
  • A tunnel passing through a sphere is homeomorphic to a tunnel similarly connecting pole to pole but going outside the sphere instead of inside
  • Now imagine deflating the sphere so it's the same width as the tunnel, giving a torus

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