(Symmetric Product)
There is a natural forgetful functor from topological commutative monoids to pointed topological spaces, where the basepoint of a commutative monoid is taken to be its unit element. The induced monad is the infinite symmetric product construction:
Here the colimit is taken over the maps , where is the basepoint.
Thus is the free topological commutative monoid on with the basepoint acting as the unit.
(Dold-Thom)
The ordinary homology, hence the standard -th homology group of a pointed connected CW-complex? is isomorphic to the -th homotopy group of the infinite symmetric product of
The Mayer-Vietoris sequence for homology is a consequence of applying to the homotopy pullback square resulting from the application of to the homotopy pushout square formed by the inclusions of the intersection, , of two subspaces and of a space into and .
The original article is
For a textbook account, see Hatcher, Sec. 4.K p. 475.
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