Number of 123-avoiding indecomposable permutations.

Terms

a(0) =1a(1) =1a(2) =3a(3) =11a(4) =38a(5) =127a(6) =423a(7) =1423a(8) =4854a(9) =16787a(10) =58776a(11) =208001a(12) =742888a(13) =2674427a(14) =9694831a(15) =35357655a(16) =129644774a(17) =477638683a(18) =1767263172a(19) =6564120401a(20) =24466267000a(21) =91482563619a(22) =343059613628