a(n) = (3*n^5 + 4*n^3 - n)/6.

Terms

a(0) =1a(1) =21a(2) =139a(3) =554a(4) =1645a(5) =4031a(6) =8631a(7) =16724a(8) =30009a(9) =50665a(10) =81411a(11) =125566a(12) =187109a(13) =270739a(14) =381935a(15) =527016a(16) =713201a(17) =948669a(18) =1242619a(19) =1605330a(20) =2048221a(21) =2583911a(22) =3226279a(23) =3990524a(24) =4893225a(25) =5952401a(26) =7187571a(27) =8619814a(28) =10271829a(29) =12167995