Number of partitions of an n-set without blocks of size 4.

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =14a(5) =47a(6) =173a(7) =702a(8) =3125a(9) =14910a(10) =76495a(11) =418035a(12) =2418397a(13) =14791597a(14) =95093612a(15) =641094695a(16) =4521228732a(17) =33250447919a(18) =254585084539