Number of length n inversion sequences avoiding the patterns 011 and 120.

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =14a(5) =42a(6) =132a(7) =431a(8) =1452a(9) =5026a(10) =17808a(11) =64398a(12) =237097a(13) =886935a(14) =3365348a(15) =12933691a(16) =50285912a(17) =197586405a(18) =783922944a(19) =3138088910a(20) =12666148475a(21) =51517864816a(22) =211047556685a(23) =870385025022