Twisted Adiabatic Limit for Complex Structures

Abstract: Given a complex manifold $X$ and a smooth positive function $\eta$ thereon, we perturb the standard differential operator $d=\partial + \bar\partial$ acting on differential forms to a first-order differential operator $D_\eta$ whose principal part is $\eta\partial + \bar\partial$. The role of the zero-th order part is to force the integrability property $D_\eta^2=0$ that leads to a cohomology isomorphic to the de Rham cohomology of $X$, while the components of types $(0,\,1)$ and $(1,\,0)$ of $D_\eta$ induce cohomologies isomorphic to the Dolbeault and conjugate-Dolbeault cohomologies. We compute Bochner-Kodaira-Nakano-type formulae for the Laplacians induced by these operators and a given Hermitian metric on $X$. The computations throw up curvature-like operators of order one that can be made (semi-)positive under appropriate assumptions on the function $\eta$. As applications, we obtain vanishing results for certain harmonic spaces on complete, non-compact, manifolds and for the Dolbeault cohomology of compact complex manifolds that carry certain types of functions $\eta$. This study continues and generalises the one of the operators $d_h=h\partial + \bar\partial$ that we introduced and investigated recently for a positive constant $h$ that was then let to converge to $0$ and, more generally, for constants $h\in\C$. The operators $d_h$ had, in turn, been adapted to complex structures from the well-known adiabatic limit construction for Riemannian foliations. Allowing now for possibly non-constant functions $\eta$ creates positivity in the curvature-like operator that stands one in good stead for various kinds of applications.