a(n) = (3*n^5 + 5*n^3)/8.

Terms

a(0) =1a(1) =17a(2) =108a(3) =424a(4) =1250a(5) =3051a(6) =6517a(7) =12608a(8) =22599a(9) =38125a(10) =61226a(11) =94392a(12) =140608a(13) =203399a(14) =286875a(15) =395776a(16) =535517a(17) =712233a(18) =932824a(19) =1205000a(20) =1537326a(21) =1939267a(22) =2421233a(23) =2994624a(24) =3671875a(25) =4466501a(26) =5393142a(27) =6467608a(28) =7706924a(29) =9129375