Sum_{0<j<k<=n} (k^4-j^4).

Terms

a(0) =15a(1) =160a(2) =830a(3) =2976a(4) =8477a(5) =20608a(6) =44604a(7) =88320a(8) =162987a(9) =284064a(10) =472186a(11) =754208a(12) =1164345a(13) =1745408a(14) =2550136a(15) =3642624a(16) =5099847a(17) =7013280a(18) =9490614a(19) =12657568a(20) =16659797a(21) =21664896a(22) =27864500a(23) =35476480a(24) =44747235a(25) =55954080a(26) =69407730