Published online by Cambridge University Press: 20 November 2018
A subset $W$ of a closed manifold
$M$ is
$K$-contractible, where
$K$ is a torus or Klein bottle if the inclusion
$W\,\to \,M$ factors homotopically through a map to
$K$. The image of
${{\pi }_{1}}\left( W \right)$ (for any base point) is a subgroup of
${{\pi }_{1}}\left( M \right)$ that is isomorphic to a subgroup of a quotient group of
${{\pi }_{1}}\left( K \right)$. Subsets of
$M$ with this latter property are called
${{\mathcal{G}}_{K}}$-contractible. We obtain a list of the closed 3-manifolds that can be covered by two open
${{\mathcal{G}}_{K}}$-contractible subsets. This is applied to obtain a list of the possible closed prime 3-manifolds that can be covered by two open
$K$-contractible subsets.