a(n) = 90*binomial(n-1,7) + 9*binomial(n-1,6).

Terms

a(0) =0a(1) =0a(2) =9a(3) =153a(4) =972a(5) =3996a(6) =12690a(7) =33858a(8) =79596a(9) =169884a(10) =335907a(11) =624195a(12) =1101672a(13) =1861704a(14) =3031236a(15) =4779108a(16) =7325640a(17) =10953576a(18) =16020477a(19) =22972653a(20) =32360724a(21) =44856900a(22) =61274070a(23) =82586790a(24) =109954260a(25) =144745380a(26) =188565975a(27) =243288279a(28) =311082768a(29) =394452432