Number of partitions of an n-set without blocks of size 3.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =11a(5) =32a(6) =113a(7) =422a(8) =1788a(9) =8015a(10) =39435a(11) =204910a(12) =1144377a(13) =6722107a(14) =41877722a(15) =273328660a(16) =1875326627a(17) =13427171644a(18) =100415636519a(19) =780856389454