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+/* MVCP, Minimum Vertex Cover Problem */
+
+/* Written in GNU MathProg by Andrew Makhorin <mao@gnu.org> */
+
+/* The Minimum Vertex Cover Problem in a network G = (V, E), where V
+   is a set of nodes, E is a set of arcs, is to find a subset V' within
+   V such that each edge (i,j) in E has at least one its endpoint in V'
+   and which minimizes the sum of node weights w(i) over V'.
+
+   Reference:
+   Garey, M.R., and Johnson, D.S. (1979), Computers and Intractability:
+   A guide to the theory of NP-completeness [Graph Theory, Covering and
+   Partitioning, Minimum Vertex Cover, GT1]. */
+
+set E, dimen 2;
+/* set of edges */
+
+set V := (setof{(i,j) in E} i) union (setof{(i,j) in E} j);
+/* set of nodes */
+
+param w{i in V}, >= 0, default 1;
+/* w[i] is weight of vertex i */
+
+var x{i in V}, binary;
+/* x[i] = 1 means that node i is included into V' */
+
+s.t. cov{(i,j) in E}: x[i] + x[j] >= 1;
+/* each edge (i,j) must have node i or j (or both) in V' */
+
+minimize z: sum{i in V} w[i] * x[i];
+/* we need to minimize the sum of node weights over V' */
+
+data;
+
+/* These data correspond to an example from [Papadimitriou]. */
+
+/* Optimal solution is 6 (greedy heuristic gives 13) */
+
+set E := a1 b1, b1 c1, a1 b2, b2 c2, a2 b3, b3 c3, a2 b4, b4 c4, a3 b5,
+         b5 c5, a3 b6, b6 c6, a4 b1, a4 b2, a4 b3, a5 b4, a5 b5, a5 b6,
+         a6 b1, a6 b2, a6 b3, a6 b4, a7 b2, a7 b3, a7 b4, a7 b5, a7 b6;
+
+end;