On the r-free values of the polynomial x^2+y^2+z^2+k

Abstract: 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.