Number of 4-block ordered bicoverings of an unlabeled n-set.

Terms

a(0) =0a(1) =0a(2) =0a(3) =16a(4) =63a(5) =162a(6) =341a(7) =636a(8) =1092a(9) =1764a(10) =2718a(11) =4032a(12) =5797a(13) =8118a(14) =11115a(15) =14924a(16) =19698a(17) =25608a(18) =32844a(19) =41616a(20) =52155a(21) =64714a(22) =79569a(23) =97020a(24) =117392a(25) =141036a(26) =168330a(27) =199680a(28) =235521a(29) =276318