The number of 4321-avoiding separable permutations of length n.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =21a(5) =73a(6) =243a(7) =785a(8) =2504a(9) =7968a(10) =25389a(11) =81033a(12) =258873a(13) =827263a(14) =2643616a(15) =8447300a(16) =26990489a(17) =86236655a(18) =275531223a(19) =880341121a(20) =2812760102a(21) =8987010878a(22) =28714292671a(23) =91744697633a(24) =293132350135a(25) =936583428475