  We study the extent to which sets A in Z/NZ, N prime, resemble sets of
integers from the additive point of view (``up to Freiman isomorphism''). We
give a direct proof of a result of Freiman, namely that if |A + A| < K|A| and
|A| < c(K)N then A is Freiman isomorphic to a set of integers. Because we avoid
appealing to Freiman's structure theorem, we get a reasonable bound: we can
take c(K) > exp(-cK^2 log K).
  As a byproduct of our argument we obtain a sharpening of the second author's
result on sets with small sumset in torsion groups. For example if A is a
subset of F_2^n, and if |A + A| < K|A|, then A is contained in a coset of a
subspace of size no more than 2^{CK^2}|A|.
