Number of set partitions of [n] such that block sizes are either 1 or 4.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =2a(5) =6a(6) =16a(7) =36a(8) =106a(9) =442a(10) =1786a(11) =6106a(12) =23596a(13) =120836a(14) =631632a(15) =2854216a(16) =13590396a(17) =81258556a(18) =510768316a(19) =2839808572a(20) =16008902296a(21) =108643656136a(22) =787965516416