'Check boundaries in the petersen graph
cheeger(G,k)=min(|bdy(S)|/|S| for |S|=k, 0<k<=|V(G)|/2)'

def test_petersen(self):

    def cheeger(G, k):
        return min(((len(nx.node_boundary(G, nn)) / k) for nn in combinations(G, k)))
    P = nx.petersen_graph()
    assert_almost_equals(cheeger(P, 1), 3.0, places=2)
    assert_almost_equals(cheeger(P, 2), 2.0, places=2)
    assert_almost_equals(cheeger(P, 3), 1.67, places=2)
    assert_almost_equals(cheeger(P, 4), 1.0, places=2)
    assert_almost_equals(cheeger(P, 5), 0.8, places=2)
