IntroductionOne of the outstanding problems in four-dimensional topology is to find the minimal genus of an oriented smoothly embedded surface representing a given homology class in a smooth four-manifold. For an arbitrary homology class in an arbitrary smooth manifold not even a conjectural lower bound is known. However, for the classes represented by smooth algebraic curves in (simply connected) algebraic surfaces, it is possible that the genus of the algebraic curve, given by the adjunction formula There are a number of results on this question in the literature. They can be divided into two classes, those proved by classical topological methods, which apply to topologically locally flat embeddings, including the G-signature theorem, and those proved by methods of gauge theory, which apply to smooth embeddings only.On the classical side, there is the result of Kervaire and Milnor[10], based on Rokhlin's theorem, which shows that certain homology classes are not represented by spheres. A major step forward was made by Rokhlin[20] and Hsiang and Szczarba [8] who introduced branched covers to study this problem for divisible homology classes. If the integral homology class represented by an embedded surface £ c: X is divisible by k, then the corresponding cover of X of order k branched along £ is again a smooth 4-manifold, so there is an obvious inequality, simply from the existence of the covering, asserting that its second Betti number is at least the absolute value of its signature. In the case k = 2, this gives