In mathematics, a cobordism (W, M, M−) of an (n + 1)-dimensionsal manifold (with boundary) W between its boundary components, two n-manifolds M and M−, is called a semi-s-cobordism if (and only if) the inclusion is a simple homotopy equivalence (as in an s-cobordism) but the inclusion is not a homotopy equivalence at all.
the original creator of this topic, Jean-Claude Hausmann, used the notation M− for the right-hand boundary of the cobordism.
A consequence of (W, M, M−) being a semi-s-cobordism is that the kernel of the derived homomorphism on fundamental groups is perfect. A corollary of this is that solves the group extension problem . The solutions to the group extension problem for proscribed quotient group and kernel group K are classified up to congruence (see Homology by MacLane, e.g.), so there are restrictions on which n-manifolds can be the right-hand boundary of a semi-s-cobordism with proscribed left-hand boundary M and superperfect kernel group K.
Relationship with Plus cobordisms
Note that if (W, M, M−) is a semi-s-cobordism, then (W, M−, M) is a Plus cobordism. (This justifies the use of M− for the right-hand boundary of a semi-s-cobordism, a play on the traditional use of M+ for the right-hand boundary of a Plus cobordism.) Thus, a semi-s-cobordism may be thought of as an inverse to Quillen's Plus construction in the manifold category. Note that (M−)+ must be diffeomorphic (respectively, piecewise-linearly (PL) homeomorphic) to M but there may be a variety of choices for (M+)− for a given closed smooth (respectively, PL) manifold M.
- MacLane (1963), Homology, pp. 124–129, ISBN 0-387-58662-8
- Hausmann, Jean-Claude (1976), "Homological Surgery", Annals of Mathematics. Second Series, 104 (3): 573–584, doi:10.2307/1970967, JSTOR 1970967.
- Hausmann, Jean-Claude; Vogel, Pierre (1978), "The Plus Construction and Lifting Maps from Manifolds", Proceedings of Symposia in Pure Mathematics, 32: 67–76.
- Hausmann, Jean-Claude (1978), "Manifolds with a Given Homology and Fundamental Group", Commentarii Mathematici Helvetici, 53 (1): 113–134, doi:10.1007/BF02566068.