Lovász-Schrijver SDP-operator, near-perfect graphs and near-bipartite graphs

We study the Lovász-Schrijver lift-and-project operator ($LS_+$) based on the cone of symmetric, positive semidefinite matrices, applied to the fractional stable set polytope of graphs. The problem of obtaining a combinatorial characterization of graphs for which the $LS_+$-operator generates the stable set polytope in one step has been open since 1990. We call these graphs $LS_+$-perfect. In the current contribution, we pursue a full combinatorial characterization of $LS_+$-perfect graphs and make progress towards such a characterization by establishing a new, close relationship among $LS_+$-perfect graphs, near-bipartite graphs and a newly introduced concept of full-support-perfect graphs.

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