>746
　|x|+|y|≦n-|z|, x+y+z-n=(偶数), を満たす格子点の数は、g(n,z) = (n+1-|z|)^2.
　f(n) = 納z=-n,n] g(n,z) = 納z=-n,n] (n+1-|z|)^2 = (n+1){1+2(n+1)^2}/3,
　2/3.