Let $k_i\in \mathbb N$ $(i\ge 1)$ satisfy $2\le k_1\le k_2\le \ldots $. Freiman's theorem shows that when $j\in \mathbb N$, there exists $s=s(j)\in \mathbb N$ such that all large integers $n$ are represented in the form $n=x_1^{ k_j}+x_2^{ k_{ j+1}}+\ldots +x_s^{ k_{ j+s-1}}$, with $x_i\in \mathbb N$, if and only if $\sum k_i^{ -1}$ diverges. We make this theorem effective by showing that, for each fixed $j$, it suffices to impose the condition \[ \sum_{ i=j}^\infty k_i^{ -1}\ge 2\log k_j +4.71. \] More is established when the sequence of exponents forms an arithmetic progression. Thus, for example, when $k\in \mathbb N$ and $s\ge 100(k+1)^2$, all large integers $n$ are represented in the form $n=x_1^k+x_2^{ k+1}+\ldots +x_s^{ k+s-1}$, with $x_i\in \mathbb N$.
Categories: math.NT