Dolgachev surface
# Dolgachev surface

In mathematics, Dolgachev surfaces are certain simply connected elliptic surfaces, introduced by Igor Dolgachev (1981). They can be used to give examples of an infinite family of homeomorphic simply connected compact 4-manifolds, no two of which are diffeomorphic.

## Properties

The blowup $X_{0}$ of the projective plane in 9 points can be realized as an elliptic fibration all of whose fibers are irreducible. A Dolgachev surface $X_{q}$ is given by applying logarithmic transformations of orders 2 and q to two smooth fibers for some $q\geq 3$.

The Dolgachev surfaces are simply connected, and the bilinear form on the second cohomology group is odd of signature $(1,9)$ (so it is the unimodular lattice $I_{1,9}$). The geometric genus $p_{g}$ is 0 and the Kodaira dimension is 1.

Simon Donaldson (1987) found the first examples of homeomorphic but not diffeomorphic 4-manifolds $X_{0}$ and $X_{3}$. More generally the surfaces $X_{q}$ and $X_{r}$ are always homeomorphic, but are not diffeomorphic unless $q=r$.

Selman Akbulut (2012) showed that the Dolgachev surface $X_{3}$ has a handlebody decomposition without 1- and 3-handles.

