ok   Hopf link: |lk| = 1 by crossings (over = under) and by the Gauss integral (1, 1, 1.000000000000)
ok   two far squares: lk = 0 (0, 0, 1.26e-17)
ok   spiral winding 2 times around an axis: |lk| = 2 (2, 2, 2.000000000000)
ok   spiral winding 3 times around an axis: |lk| = 3 (3, 3, 3.000000000000)
ok   random polygons: two projections, over = under, and the Gauss integral agree 299 of 299; values of |lk|: [(0, 207), (1, 82), (2, 10)]
ok   connected subsets containing a vertex: equal to brute force on 40 random graphs 
ok   partitions of K_8 into k blocks are counted by Stirling numbers {3: 966, 5: 1050, 6: 266}
ok   Petersen family: seven graphs with 15 edges on 6, 7, 7, 8, 8, 9, 10 vertices [(6, 15), (7, 15), (7, 15), (8, 15), (8, 15), (9, 15), (10, 15)]
ok   the family contains K_6, K_{3,3,1}, K_{4,4} minus an edge and the Petersen graph 
ok   each of the seven graphs is found as a minor of itself 
ok   no graph of the family minus an edge has a minor in the family (105 graphs, exhaustive) 
ok   K_6: the pruned search and the general partition search agree on random graphs (13 with a minor)
ok   every random straight-line embedding of each of the seven graphs has a pair with odd linking number (28 embeddings); the number of such pairs is odd 
ok   f-vectors of the simplex, the cube, the cross-polytope and C(n,4), n = 6, 7, 8 {'simplex': (5, 10, 10, 5), 'cube': (16, 32, 24, 8), 'cross': (8, 24, 32, 16), 'C(6,4)': (6, 15, 18, 9), 'C(7,4)': (7, 21, 28, 14), 'C(8,4)': (8, 28, 40, 20)}
ok   points that are not vertices are dropped 
ok   the polar of the cube is the cross-polytope 
ALL OK
