a(n) = binomial(n,6)-1.

Terms

a(0) =0a(1) =6a(2) =27a(3) =83a(4) =209a(5) =461a(6) =923a(7) =1715a(8) =3002a(9) =5004a(10) =8007a(11) =12375a(12) =18563a(13) =27131a(14) =38759a(15) =54263a(16) =74612a(17) =100946a(18) =134595a(19) =177099a(20) =230229a(21) =296009a(22) =376739a(23) =475019a(24) =593774a(25) =736280a(26) =906191a(27) =1107567a(28) =1344903a(29) =1623159