21*n^4 - 36*n^3 + 25*n^2 - 8*n + 1.

Terms

a(0) =1a(1) =3a(2) =133a(3) =931a(4) =3441a(5) =9211a(6) =20293a(7) =39243a(8) =69121a(9) =113491a(10) =176421a(11) =262483a(12) =376753a(13) =524811a(14) =712741a(15) =947131a(16) =1235073a(17) =1584163a(18) =2002501a(19) =2498691a(20) =3081841a(21) =3761563a(22) =4547973a(23) =5451691a(24) =6483841a(25) =7656051a(26) =8980453a(27) =10469683a(28) =12136881a(29) =13995691