Let Q(x) = Q(x_1, x_2, ..., x_n) be an integer-valued n-ary quadratic form, i.e., a polynomial with integer coefficients which satisfies Q(x)>0 for real x!=0. Then Q(x) can be represented by Q(x) = x^T A x, where A = 1/2 (d^2 Q(x))/(dx_i dx_j)