G.f. is the polynomial (Product_{k=1..20} (1 - x^(3*k)))/(1-x)^20.

Terms

a(0) =1a(1) =20a(2) =210a(3) =1539a(4) =8835a(5) =42294a(6) =175559a(7) =648925a(8) =2177361a(9) =6728260a(10) =19363355a(11) =52364721a(12) =134036525a(13) =326685790a(14) =761961825a(15) =1707940096a(16) =3692525360a(17) =7724060310a(18) =15675545395a(19) =30937970105a(20) =59507001114a(21) =111753986081