Construction of symplectic solvmanifolds satisfying the hard-Lefschetz condition

Abstract: A compact symplectic manifold $(M, \omega)$ is said to satisfy the hard-Lefschetz condition if it is possible to develop an analogue of Hodge theory for $(M, \omega)$. This loosely means that there is a notion of harmonicity of differential forms in $M$, depending on $\omega$ alone, such that every de Rham cohomology class in has a $\omega$-harmonic representative. In this article, we study two non-equivalent families of diagonal almost-abelian Lie algebras that admit a distinguished almost-K\"ahler structure and compute their cohomology explicitly. We show that they satisfy the hard-Lefschetz condition with respect to any left-invariant symplectic structure by exploiting an unforeseen connection with Kneser graphs. We also show that for some choice of parameters their associated simply connected, completely solvable Lie groups admit lattices, thereby constructing examples of almost-K\"ahler solvmanifolds satisfying the hard-Lefschetz condition, in such a way that their de Rham cohomology is fully known.