We prove that the Dolgachev surface E(1)_{2,3} (which is an exotic copy of
the elliptic surface E(1)=CP^2 # 9(-CP^2)) can be obtained from E(1) by
twisting along a simple "plug", in particular it can be obtained from E(1) by
twisting along an RP^2.
We show that an infinite sequence of homotopy 4-spheres constructed by
Cappell-Shaneson are all diffeomorphic to S^4. This generalizes previous
results of Akbulut-Kirby and Gompf.
Previously the two of the authors defined a notion of dual Calabi-Yau
manifolds in a G_2 manifold, and described a process to obtain them. Here we
apply this process to a compact G_2 manifold, constructed by Joyce, and as a
result we obtain a pair of Borcea-Voisin Calabi-Yau manifolds, which are known
to be mirror duals of each other.
It is known that the only Stein filling of the standard contact structure on
S^3 is B^4. In this paper, we construct simply connected exotic compact Stein
4-manifold pairs for any Betti number $b_2 \geq 1$; we do this by enlarging
corks and plugs.
It is known that every exotic smooth structure on a simply connected closed
4-manifold is determined by a codimention zero compact contractible Stein
submanifold and an involution on its boundary. Such a pair is called a cork. In
this paper, we construct infinitely many knotted imbeddings of corks in
4-manifolds such that they induce infinitely many different exotic smooth
structures.