One of the most puzzling
questions in low dimensional topology is which elements α ∈ π2(M), where M is a
smooth compact 4-manifold, may be represented by a smoothly imbedded 2-sphere.
This paper treats a stable version of the problem: When is there a smooth proper
imbedding, h : S2×R↪M × R by which the ends of S2×R are mapped to the ends
of M × R, and for which the composition