Abstract. We prove that bordism group of spin 4-manifolds with singular Tstructure, the notion introduced by Cheeger and Gromov, is an infinite cyclic group and is detected by singnature. In particular we obtain that the theorem of Atiyah and Hirzebruch of vanishing ofÂ-genus in case of S 1 action on spin 4n-manifolds is not valid in case of T -structures on spin 4-manifolds.
IntroductionThe notion of a local action of tori on a manifold is a generalization of an action of a torus. Tori, possibly of different dimensions, act effectively on open subsets of the manifold and these actions fit together on overlaps in such a way that the torus acting on one of the sets injects homomorphically into the torus acting on the second one. If we assume that each of these actions is without fixed points, then the local action of tori coincides with a T -structure. The notion of a T -structure and more general notions of an F structure and a nilpotent Killing structure appeared [CFG] in the context of collapsing Riemannian manifolds.Here we assume that the manifold is compact, differentiable, spin and that the local action of tori may admit fixed points. The main result of the paper is
Theorem. The bordism group of compact spin 4-manifolds admitting local actions of tori is isomorphic to Z. There is a generator with signature 16.In particular we obtain that any compact spin 4-manifold is spin cobordant with a manifold admitting local action of tori. In dimension 4 Sign(M ) = − According to the present knowledge of the author the statement of the conjecture is true for pure local action of tori on 4-manifolds and under certain assumptions on the dimension of the acting torus for 8-manifolds.