G.f. is the polynomial (Product_{k=1..18} (1 - x^(3*k)))/(1-x)^18.

Terms

a(0) =1a(1) =18a(2) =171a(3) =1139a(4) =5967a(5) =26163a(6) =99806a(7) =340101a(8) =1055070a(9) =3022463a(10) =8084196a(11) =20366271a(12) =48670438a(13) =110977461a(14) =242626770a(15) =510702236a(16) =1038606993a(17) =2046949686a(18) =3919950135a(19) =7310939625a(20) =13306606299a(21) =23677953958a(22) =41257164222a(23) =70494485616