On the dimension of Dolbeault harmonic (1,1)-forms on almost Hermitian 4-manifolds

Abstract

We prove that the dimension $h^{1,1}_{\overline\partial}$ of the space of Dolbeault harmonic $(1,1)$-forms is not necessarily always equal to $b^-$ on a compact almost complex 4-manifold endowed with an almost Hermitian metric which is not locally conformally almost K\"ahler. Indeed, we provide examples of non integrable, non locally conformally almost K\"ahler, almost Hermitian structures on compact 4-manifolds with $h^{1,1}_{\overline\partial}=b^-+1$. This answers to a question by Holt.