G.f. is the polynomial (Product_{k=1..21} (1 - x^(3*k)))/(1-x)^21.

Terms

a(0) =1a(1) =21a(2) =231a(3) =1770a(4) =10605a(5) =52899a(6) =228458a(7) =877383a(8) =3054744a(9) =9783004a(10) =29146359a(11) =81511080a(12) =215547605a(13) =542233395a(14) =1304195220a(15) =3012135316a(16) =6704660676a(17) =14428720986a(18) =30104266381a(19) =61042236486a(20) =120549237600a(21) =232303223681