a(n) = 3^n - n^7.

Terms

a(0) =1a(1) =2a(2) =-119a(3) =-2160a(4) =-16303a(5) =-77882a(6) =-279207a(7) =-821356a(8) =-2090591a(9) =-4763286a(10) =-9940951a(11) =-19310024a(12) =-35300367a(13) =-61154194a(14) =-100630535a(15) =-156510468a(16) =-225388735a(17) =-281198510a(18) =-224799543a(19) =268389728a(20) =2206784401