a(n) = Sum_{d|n, d==1 mod 4} d^7 - Sum_{d|n, d==3 mod 4} d^7.

Terms

a(0) =1a(1) =1a(2) =-2186a(3) =1a(4) =78126a(5) =-2186a(6) =-823542a(7) =1a(8) =4780783a(9) =78126a(10) =-19487170a(11) =-2186a(12) =62748518a(13) =-823542a(14) =-170783436a(15) =1a(16) =410338674a(17) =4780783a(18) =-893871738a(19) =78126a(20) =1800262812a(21) =-19487170a(22) =-3404825446a(23) =-2186a(24) =6103593751a(25) =62748518a(26) =-10455572420a(27) =-823542