Number of permutations with certain forbidden subsequences.

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =14a(5) =41a(6) =123a(7) =374a(8) =1147a(9) =3538a(10) =10958a(11) =34042a(12) =105997a(13) =330632a(14) =1032781a(15) =3229714a(16) =10109310a(17) =31667245a(18) =99260192a(19) =311294876a(20) =976709394a(21) =3065676758a(22) =9625674442a(23) =30231524869a(24) =94972205349a(25) =298419158008a(26) =937861780439