Intermediate-Range Estimates for Short Weyl Sums and Waring's Problem with Almost Proportional Summands

Source abstract

We obtain a uniform pointwise estimate for short Weyl sums of degree $n\geq3$ in the intermediate rational-approximation range $$\frac{1}{qx^{n-2}y}\ll|λ| \ll\frac{1}{qy^{n-1}}.$$ This range arises from a second application of Dirichlet's rational approximation theorem in estimating the residual integral that occurs in the derivation of an asymptotic formula for Waring's problem with almost proportional summands. For $r=2^n+1$ and fixed positive numbers $μ_1,\ldots,μ_r$ satisfying $$μ_1+\cdots+μ_r=1,$$ we derive an asymptotic formula for the number of representations $$x_1^n+\cdots+x_r^n=N, \qquad |x_i^n-μ_iN|\leq H,\qquad 1\le i \le r,$$ valid for $$N^{1-θ(n,r)+\varepsilon}\le H \le \frac{N}{\ln N}, \quad θ(n,r)= \frac{2}{n\bigl((r-1)(n-1)+2\bigr)}.$$ The resulting admissible lower bound for $H$ improves the previously known bound for every $n\geq3$.