G.f. A(x) satisfies: A(x)^3 = A(x^3) / (1 - 3*x).

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =12a(5) =31a(6) =83a(7) =224a(8) =615a(9) =1708a(10) =4777a(11) =13455a(12) =38110a(13) =108428a(14) =309714a(15) =887666a(16) =2551575a(17) =7353423a(18) =21240460a(19) =61478489a(20) =178269670a(21) =517784717a(22) =1506162369a(23) =4387201004a(24) =12795170784a(25) =37359689295a(26) =109199349181a(27) =319493390481a(28) =935616592227