a(n) = ((n + 1) - 9*(n + 1)^2 + 8*(n + 1)^3)/6.

Terms

a(0) =0a(1) =5a(2) =23a(3) =62a(4) =130a(5) =235a(6) =385a(7) =588a(8) =852a(9) =1185a(10) =1595a(11) =2090a(12) =2678a(13) =3367a(14) =4165a(15) =5080a(16) =6120a(17) =7293a(18) =8607a(19) =10070a(20) =11690a(21) =13475a(22) =15433a(23) =17572a(24) =19900a(25) =22425a(26) =25155a(27) =28098a(28) =31262a(29) =34655