# A075358 (b-file synthesized from sequence entry) 1 1 2 3 3 20 4 30 5 336 6 5040 7 7920 8 154440 9 3603600 10 5765760 11 160392960 12 5079110400 13 8204716800 14 296541907200 15 11861676288000 16 520431047136000 17 857180548224000 18 41430393164160000 19 2163102632570880000 20 3569119343741952000