Paper detail

Lattices with lots of congruence energy

In 1978, motivated by E. Hückel's work in quantum chemistry, I. Gutman introduced the concept of the energy of a finite simple graph $G$ as the sum of the absolute values of the eigenvalues of the adjacency matrix of $G$. At the time of writing, the MathSciNet search for "Title=(graph energy) AND Review Text=(eigenvalue)" returns 351 publications, most of which going after Gutman's definition. A congruence $α$ of a finite algebra $A$ turns $A$ into a simple graph: we connect $x\neq y\in A$ by an edge iff $(x,y)\inα$; we let En$(α)$ be the energy of this graph. We introduce the congruence energy CE$(A)$ of $A$ by CE$(A):=\sum\{$En$(α): α\in$ Con$(A)\}$. Let LAT$(n)$ and CDA$(n)$ stand for the class of $n$-element lattices and that of $n$-element congruence distributive algebras of any type. For a class $\mathcal X$, let CE$(\mathcal X):= \{$CE$(A): A\in \mathcal X\}$. We prove the following. (1) For $α\in A$, En$(α)/2$ is the height of $α$ in the equivalence lattice of $A$. (2) The largest number and the second largest number in CE(LAT($n$)) are $(n-1)\cdot 2^{n-1}$ and, for $n\geq 4$, $(n-1)\cdot 2^{n-2}+2^{n-3}$; these numbers are only witnessed by chains and lattices with exactly one two-element antichain, respectively. (3) The largest number in CE(CDA($n$)) is also $(n-1)\cdot 2^{n-1}$, and if CE$(A)=(n-1)\cdot 2^{n-1}$ for an $A\in$ CDA$(n)$, then Con$(A)$ is a boolean lattice with size $|$Con$(A)|=2^{n-1}$.