The number of 654321-avoiding separable permutations of length n.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =22a(5) =90a(6) =393a(7) =1769a(8) =7957a(9) =35133a(10) =151675a(11) =642695a(12) =2689411a(13) =11176469a(14) =46313531a(15) =191837707a(16) =795251170a(17) =3300506324a(18) =13712825121a(19) =57019988099a(20) =237221971144a(21) =987206194720