Classification of real Bott manifolds and acyclic digraphs

TL;DR

This paper provides a complete classification of real Bott manifolds using matrix and graph operations, establishes their cohomology ring isomorphisms, and confirms the toral rank conjecture for these manifolds.

Contribution

It introduces a new combinatorial characterization of real Bott manifolds via matrix and graph operations, and proves the uniqueness of their decomposition.

Findings

Classification of real Bott manifolds via matrix operations

Cohomology ring isomorphisms induced by affine diffeomorphisms

Verification of the toral rank conjecture for real Bott manifolds

Abstract

We completely characterize real Bott manifolds up to affine diffeomorphism in terms of three simple matrix operations on square binary matrices obtained from strictly upper triangular matrices by permuting rows and columns simultaneously. We also prove that any graded ring isomorphism between the cohomology rings of real Bott manifolds with $\mathbb Z/2$ coefficients is induced by an affine diffeomorphism between the real Bott manifolds. Our characterization can also be described in terms of graph operations on directed acyclic graphs. Using this combinatorial interpretation, we prove that the decomposition of a real Bott manifold into a product of indecomposable real Bott manifolds is unique up to permutations of the indecomposable factors. Finally, we produce some numerical invariants of real Bott manifolds from the viewpoint of graph theory and discuss their topological meaning. As a by-product, we prove that the toral rank conjecture holds for real Bott manifolds.