  Let k be a fixed integer. We study the asymptotic formula of R(H, r, k),
which is the number of positive integer solutions x, y, z greater than or equal
to 1 and less than or equal to H such that the polynomial x^2+y^2+z^2+k is
r-free. We obtained the asymptotic formula of R(H, r, k) for all r greater than
or equal to 2. Our result is new even in the case r = 2. We proved that R(H, 2,
k) = ckH^3 + O(H^(9/4+epsilon)), where ck > 0 is a constant depending on k.
This improves upon the error term O(H^(7/3+epsilon)) obtained by Zhou and Ding.
