a(n) = 1 - n^7.

Terms

a(0) =1a(1) =0a(2) =-127a(3) =-2186a(4) =-16383a(5) =-78124a(6) =-279935a(7) =-823542a(8) =-2097151a(9) =-4782968a(10) =-9999999a(11) =-19487170a(12) =-35831807a(13) =-62748516a(14) =-105413503a(15) =-170859374a(16) =-268435455a(17) =-410338672a(18) =-612220031a(19) =-893871738a(20) =-1279999999