Compact complex surfaces with geometric structures related to split   quaternions
  We study the problem of existence of geometric structures on compact complex surfaces that are related to split quaternions. These structures, called para-hypercomplex, para-hyperhermitian and para-hyperk\"ahler are analogs of the hypercomplex, hyperhermitian and hyperk\"ahler structures in the definite case. We show that a compact oriented 4-manifold carries a para-hyperk\"ahler structure iff it has a metric of split signature together with two parallel, orthogonal and null vector fields. Every compact complex surface admiting a para-hyperhermitian structure has vanishing first Chern class and we show that, unlike the definite case, many of these surfaces carry infinite dimensional families of such structures. We provide also compact examples of complex surfaces with para-hyperhermitian structures which are not locally conformally para-hyperk\"ahler. Finally, we discuss the problem of non-existence of para-hyperhermitian structures on Inoue surfaces of type $S^0$ and provide a list of compact complex surfaces which could carry para-hypercomplex structures. 
Johann Davidov, Gueo Grantcharov, Oleg Mushkarov, Miroslav Yotov
math.DG hep-th