a(n) = Sum_{-n<i<n, -n<j<n, gcd{i,j}=3} (n-|i|)*(n-|j|)/4.

Terms

a(0) =0a(1) =0a(2) =0a(3) =5a(4) =14a(5) =27a(6) =52a(7) =85a(8) =126a(9) =197a(10) =284a(11) =387a(12) =534a(13) =705a(14) =900a(15) =1187a(16) =1514a(17) =1881a(18) =2328a(19) =2823a(20) =3366a(21) =4095a(22) =4896a(23) =5769a(24) =6818a(25) =7955a(26) =9180a(27) =10667a(28) =12266a(29) =13977