Cover the circle by two arcs whose intersection has two components and take the product with the projective plane. The Mayer–Vietoris sequence has maps on the two copies of the homology of the plane. Its kernels and cokernels, together with , give the displayed groups; all higher groups vanish. The product is a closed nonorientable three-manifold, but is homotopy equivalent to a noncompact oriented four-manifold.
Articles by others on the same topic
There are currently no matching articles.