# Sums and differences of four *k*th powers

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## Abstract

We prove an upper bound for the number of representations of a positive integer *N* as the sum of four *k*th powers of integers of size at most *B*, using a new version of the determinant method developed by Heath-Brown, along with recent results by Salberger on the density of integral points on affine surfaces. More generally we consider representations by any integral diagonal form. The upper bound has the form \({O_{N}(B^{c/\sqrt{k}})}\), whereas earlier versions of the determinant method would produce an exponent for *B* of order *k* ^{−1/3} (uniformly in *N*) in this case. Furthermore, we prove that the number of representations of a positive integer *N* as a sum of four *k*th powers of non-negative integers is at most \({O_{\varepsilon}(N^{1/k+2/k^{3/2}+\varepsilon})}\) for *k* ≥ 3, improving upon bounds by Wisdom.

### Keywords

Sum of*k*th powers Determinant method Diagonal form Integral points

### Mathematics Subject Classification (2000)

11D85 14G05## Preview

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