G.f. A(x) satisfies A(x) = 1 + x/(1-x^3)^3 * A(x)^2.

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =17a(5) =54a(6) =177a(7) =603a(8) =2102a(9) =7463a(10) =26907a(11) =98258a(12) =362683a(13) =1350990a(14) =5072190a(15) =19174068a(16) =72919800a(17) =278797240a(18) =1071004920a(19) =4131821244a(20) =16001364200a(21) =62184444840a(22) =242426757780a(23) =947841072984