An invariant of smooth 4-manifolds
Abstract
We define a diffeomorphism invariant of smooth 4-manifolds which we can estimate for many smoothings of R4 and other smooth 4-manifolds. Using this invariant we can show that uncountably many smoothings of R4 support no Stein structure. (Gompf has constructed uncountably many smoothings of R4 which do support Stein structures.) Other applications of this invariant are given.
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