a(n) = Sum_{i=1..n, j=1..n, gcd(i,j)=2} (n+1-i)*(n+1-j).

Terms

a(0) =0a(1) =1a(2) =4a(3) =15a(4) =32a(5) =71a(6) =124a(7) =211a(8) =320a(9) =499a(10) =716a(11) =999a(12) =1328a(13) =1799a(14) =2340a(15) =3023a(16) =3792a(17) =4767a(18) =5852a(19) =7135a(20) =8544a(21) =10319a(22) =12260a(23) =14471a(24) =16864a(25) =19775a(26) =22916a(27) =26467a(28) =30272a(29) =34587