a(n) = n*(n^7 + 1)/2.

Terms

a(0) =0a(1) =1a(2) =129a(3) =3282a(4) =32770a(5) =195315a(6) =839811a(7) =2882404a(8) =8388612a(9) =21523365a(10) =50000005a(11) =107179446a(12) =214990854a(13) =407865367a(14) =737894535a(15) =1281445320a(16) =2147483656a(17) =3487878729a(18) =5509980297a(19) =8491781530a(20) =12800000010a(21) =18911429691a(22) =27437936779a(23) =39155492652a(24) =55037657100a(25) =76293945325