Some inequalities for orderings of acyclic digraphs


  • Thomas Bier
  • Imed Zaguia Royal Military College of Canada



(partially) ordered set, digraph, acyclic ordering, linear extension, order dimension two


Let $D=(V,A)$ be an acyclic digraph. For $x\in V$ define $e_{_{D}}(x)$ to be the difference of the indegree and the outdegree of $x$. An acyclic ordering of the vertices of $D$ is a one-to-one map $g: V \rightarrow [1,|V|] $ that has the property that for all $x,y\in V$ if $(x,y)\in A$, then $g(x) < g(y)$.

We prove that for every acyclic ordering $g$ of $D$ the following inequality holds:
\[\sum_{x\in V} e_{_{D}}(x)\cdot g(x) ~\geq~ \frac{1}{2} \sum_{x\in V}[e_{_{D}}(x)]^2~.\]
The class of acyclic digraphs for which equality holds is determined as the class of comparability digraphs of posets of order dimension two.


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