  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 $x_i\in \mathbb N$.
