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12 votes

Homology of Covering Spaces

This is not a trivial problem. A favorite example of mine is the case of a knot complement, $S^3\setminus K$. (It is known that these are Eilenberg-Maclane spaces.) If you pick $A$ to be the commutato …
Jim Conant's user avatar
  • 4,898
5 votes

Fundamental groups of surfaces

The word problem for the fundamental group of a closed surface is solvable, using Dehn's algorithm. Since any finitely presented group appears as the fundamental group of some closed $4$-manifold, and …
Jim Conant's user avatar
  • 4,898