G.f. is the polynomial (1-x^3) * (1-x^6) * (1-x^9) * (1-x^12) * (1-x^15) * (1-x^18) / (1-x)^6.

Terms

a(0) =1a(1) =6a(2) =21a(3) =55a(4) =120a(5) =231a(6) =405a(7) =660a(8) =1014a(9) =1484a(10) =2085a(11) =2829a(12) =3724a(13) =4773a(14) =5973a(15) =7315a(16) =8784a(17) =10359a(18) =12013a(19) =13713a(20) =15420a(21) =17091a(22) =18681a(23) =20145a(24) =21440a(25) =22527a(26) =23373a(27) =23952a(28) =24246a(29) =24246