Number of 7-block ordered bicoverings of an unlabeled n-set.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =1680a(6) =27342a(7) =208302a(8) =1099602a(9) =4636072a(10) =16734438a(11) =53810484a(12) =158053119a(13) =431305959a(14) =1106791524a(15) =2694914978a(16) =6269281305a(17) =14010246285a(18) =30208869495a(19) =63074014815a(20) =127909521180a(21) =252581107180a(22) =486738385140