Number of compositions of n with parts in N which avoid the pattern 221.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =8a(5) =15a(6) =30a(7) =58a(8) =113a(9) =220a(10) =429a(11) =835a(12) =1627a(13) =3169a(14) =6172a(15) =12023a(16) =23419a(17) =45616a(18) =88853a(19) =173073a(20) =337118a(21) =656656a(22) =1279065a(23) =2491423a(24) =4852911a(25) =9452731a(26) =18412473a(27) =35864686a(28) =69858930a(29) =136074521