“…If this is the case one has 1 = χ(O S ) = 1 − q + p g and it follows that p g = q. If we also assume that the surface is irregular (i.e., q > 0) then the Bogomolov-Miyaoka-Yau and Debarre inequalities, K 2 S ≤ 9, K 2 S ≥ 2 p g , imply 1 ≤ p g ≤ 4, see [6]. If p g = q = 4 we have a product of curves of genus 2, as shown by Beauville in the appendix to [21], while the case p g = q = 3 was understood through the work of several authors [18,26,32].…”