Sum{gcd(k^2 + t^2, n) * cos(2*Pi*(k^2 + t^2)/n): 0<k,t<=n}.

Terms

a(0) =1a(1) =2a(2) =-1a(3) =8a(4) =41a(5) =-2a(6) =-1a(7) =32a(8) =81a(9) =82a(10) =-1a(11) =-8a(12) =313a(13) =-2a(14) =-41a(15) =128a(16) =545a(17) =162a(18) =-1a(19) =328a(20) =1a(21) =-2a(22) =-1a(23) =-32a(24) =1425a(25) =626a(26) =-81a(27) =-8a(28) =1625a(29) =-82