# Waring problem

2010 Mathematics Subject Classification: Primary: 11P05 [MSN][ZBL]

A problem in number theory formulated in 1770 by E. Waring in the following form: Any natural number is a sum of 4 squares, of 9 cubes and of 19 fourth-powers. In other words, for all $k\geq2$ there exists a $s=s(k)$, depending only on $k$, such that every natural number is the sum of $s$ $k$-th powers of non-negative integers. D. Hilbert in 1909 was the first to give a general solution of Waring's problem with a very rough estimate of the value of $s$ as a function of $k$; this is why the problem is sometimes known as the Hilbert–Waring problem. Let $J_{s,k}(N)$ be the number of solutions of the equation

\begin{equation}\label{war}x_1^k+\cdots+x_s^k=N\end{equation}

in non-negative integers. Hilbert's theorem then states that there exists a $s=s(k)$ for which $J_{s,k}(N)\geq1$ for any $N\geq1$. G.H. Hardy and J.E. Littlewood, who applied the circle method to the Waring problem, demonstrated in 1928 that for $s\geq(k-2)2^{k-1}+5$ the value of $J_{s,k}(N)$ is given by an asymptotic formula of the type

\begin{equation}\label{asym}J_{s,k}(N)=AN^{s/k-1}+O(N^{s/k-1-\gamma}),\end{equation}

where $A=A(N)\geq c_0>0$, while $c_0$ and $\gamma>0$ are constants. Consequently, if $N\geq N_0(k)$, equation \ref{war} has a solution.

An elementary proof of Waring's problem was given in 1942 by Yu. V. Linnik. There exist many different generalizations of Waring's problem (the variables run through a certain subset of the set of natural numbers; the number $N$ is represented by polynomials $f_1(x_1),\ldots,f_s(x_s)$ rather than by monomials $x_1^k,\ldots,x_s^k$; equation (1) is replaced by a congruence, etc.).

Research on Waring's problem has mainly focused on sharpening estimates for the following three questions:

1. Find the smallest $s$ such that \ref{war} has solutions for all sufficiently large $N$;
2. Find the smallest $s$ such that \ref{war} has solutions for all $N$;
3. Find the smallest $s$ such that the number of solutions to \ref{war}, $J_{s,k}(N)$, is given by the asymptotic formula \ref{asym}.

These quantities are known as $G(k)$, $g(k)$, and $\tilde{G}(k)$ respectively. Clearly, $\tilde{G}(k)\geq G(k)$ and $g(k)\geq G(k)$. The progress on bounds for these quantities is detailed below.

## Solvable for $N$ sufficiently large

Let $G(k)$ be the smallest integer such that equation \ref{war} is solvable for $s\geq G(k)$ and $N$ sufficiently large depending on $k$.

It is known that $G(k)\geq k+1$. It was proved in 1934 by I.M. Vinogradov, using his own method, that

$$G(k)\leq 3k(\ln k+9).$$

Moreover, many results are available concerning $G(k)$ for small values of $k$: $G(4)=16$ (H. Davenport, 1939); $G(3)=7$ (Yu.V. Linnik, 1942).

## Solvable for all $N$

Let $g(k)$ be the smallest integer such that equation \ref{war} is solvable for $s\geq g(k)$ and $N\geq1$.

It was shown in 1936 by L. Dickson and S. Pillai, who also used the Vinogradov method, that

$$g(k)=2^k+\left[\left(\frac{3}{2}\right)^k\right]-2$$

for all $k>6$ for which

$$\left(\frac{3}{2}\right)^k-\left[\left(\frac{3}{2}\right)^k\right]\leq1-\left(\frac{1}{2}\right)^k\left\{\left[\left(\frac{3}{2}\right)^k\right]+2\right\}.$$

The last condition was demonstrated in 1957 by K. Mahler for all sufficiently large $k$.

It is known that $g(2)=4$ (J.L. Lagrange, 1770), $g(3)=9$ (A. Wieferich, A. Kempner, 1912), $g(4)=19$ (R. Balusabramanian, J. Deshouillers, F. Dress, 1986), $g(5)=37$ (Chen-Jingrun, 1964). See also Circle method and [HaWr][Sh].

## Asymptotic formula

Let $\tilde{G}(k)$ be the smallest integer such that the asymptotic formula \ref{asym} applies to $J_{s,k}(N)$ if $s\geq \tilde{G}(k)$. The result of Hardy and Littlewood mentioned above shows that

$$\tilde{G}(k)\leq(k-2)2^{k-1}+5.$$

The first substantial improvement for large values of $k$ was obtained by Vinogradov, who showed that

$$\tilde{G}(k)\leq 4k^2\ln k.$$

The current best bound for large values of $k$ was obtained by Wooley who showed that

$$\tilde{G}(k)\leq 2k^2-k^{4/3}+O(k).$$