Nilmanifolds with non-nilpotent complex structures and their pseudo-K\"ahler geometry

TL;DR

This paper classifies 8-dimensional nilpotent Lie algebras with certain complex structures, identifies those supporting pseudo-Kähler metrics, and provides new examples and topological restrictions for such geometric structures.

Contribution

It completes the classification of 8-dimensional nilpotent Lie algebras with non-nilpotent complex structures and identifies new pseudo-Kähler examples, including counterexamples to previous conjectures.

Findings

Classified nilpotent Lie algebras with weakly non-nilpotent complex structures in dimension eight.

Identified nilpotent Lie algebras supporting pseudo-Kähler metrics and provided new counterexamples.

Established a topological restriction $b_1(X) \,\geq\, 3$ for pseudo-Kähler nilmanifolds with invariant complex structures.

Abstract

We classify nilpotent Lie algebras with complex structures of weakly non-nilpotent type in real dimension eight, which is the lowest dimension where they arise. Our study, together with previous results on strongly non-nilpotent structures, completes the classification of 8-dimensional nilpotent Lie algebras admitting complex structures of non-nilpotent type. As an application, we identify those that support a pseudo-K\"ahler metric, thus providing new counterexamples to a previous conjecture and an infinite family of (Ricci-flat) non-flat neutral Calabi-Yau structures. Moreover, we arrive at the topological restriction $b_1(X)\geq 3$ for every pseudo-K\"ahler nilmanifold $X$ with invariant complex structure, up to complex dimension four.