Examples are given to show that some compact contractible 4-manifolds can be knotted in the 4-sphere. It is then proved that any finitely presented perfect group is a knot group for an embedding of some contractible 4-manifold in S.

There are known infinite families of Brieskorn homology 3-spheres which can be realized as boundaries of smooth contractible 4-manifolds. In this paper we show that smooth free periodic actions on these Brieskorn spheres do not extend smoothly over a contractible 4-manifold. We give a new infinite family of examples in which the actions extend locally linearly but not smoothly.

We show that if K is a nilpotent finite complex, then ΩK can be built from spheres using fibrations and homotopy (inverse) limits. This is applied to show that if map∗(X,S ) is weakly contractible for all n, then map ∗ (X,K) is weakly contractible for any nilpotent finite complex K . AMS Classification numbers Primary: 55Q05 Secondary: 55P50

let $r$ be a right artinian ring or a perfect commutative‎‎ring‎. ‎let $m$ be a noncosingular self-generator $sum$-lifting‎‎module‎. ‎then $m$ has a direct decomposition $m=oplus_{iin i} m_i$‎,‎where each $m_i$ is noetherian quasi-projective and each‎‎endomorphism ring $end(m_i)$ is local‎.