The number of 54321-avoiding separable permutations of length n.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =22a(5) =89a(6) =368a(7) =1488a(8) =5831a(9) =22311a(10) =84223a(11) =316181a(12) =1185884a(13) =4452567a(14) =16742230a(15) =63025805a(16) =237423928a(17) =894681874a(18) =3371727204a(19) =12706639594a(20) =47884046357a(21) =180440982667a(22) =679939553548