a(n) = Sum_{-n<i<n, -n<j<n, gcd{i,j}=3} (n-|i|)*(n-|j|).

Terms

a(0) =0a(1) =0a(2) =0a(3) =20a(4) =56a(5) =108a(6) =208a(7) =340a(8) =504a(9) =788a(10) =1136a(11) =1548a(12) =2136a(13) =2820a(14) =3600a(15) =4748a(16) =6056a(17) =7524a(18) =9312a(19) =11292a(20) =13464a(21) =16380a(22) =19584a(23) =23076a(24) =27272a(25) =31820a(26) =36720a(27) =42668a(28) =49064a(29) =55908