A strong formulation for the graph partition problem

We develop a polynomial size extended graph formulation of the graph partition problem which dominates the formulation introduced by Chopra and Rao's study, and show that the extended graph formulation is tight on a tree. We introduce exponentially many valid inequalities to the Chopra-Rao formulation, which we call generalized arc inequalities (GAI), and develop a linear time algorithm to separate the most violated generalized arc inequality. We show that the polynomial size extended graph formulation is equivalent to the Chopra-Rao formulation augmented by the exponentially many GAI.