  Separable codes were defined by Cheng and Miao in 2011, motivated by
applications to the identification of pirates in a multimedia setting.
Combinatorially, $\overline{t}$-separable codes lie somewhere between
$t$-frameproof and $(t-1)$-frameproof codes: all $t$-frameproof codes are
$\overline{t}$-separable, and all $\overline{t}$-separable codes are
$(t-1)$-frameproof. Results for frameproof codes show that (when $q$ is large)
there are $q$-ary $\overline{t}$-separable codes of length $n$ with
approximately $q^{\lceil n/t\rceil}$ codewords, and that no $q$-ary
$\overline{t}$-separable codes of length $n$ can have more than approximately
$q^{\lceil n/(t-1)\rceil}$ codewords.
  The paper provides improved probabilistic existence results for
$\overline{t}$-separable codes when $t\geq 3$. More precisely, for all $t\geq
3$ and all $n\geq 3$, there exists a constant $\kappa$ (depending only on $t$
and $n$) such that there exists a $q$-ary $\overline{t}$-separable code of
length $n$ with at least $\kappa q^{n/(t-1)}$ codewords for all sufficiently
large integers $q$. This shows, in particular, that the upper bound (derived
from the bound on $(t-1)$-frameproof codes) on the number of codewords in a
$\overline{t}$-separable code is realistic.
  The results above are more surprising after examining the situation when
$t=2$. Results due to Gao and Ge show that a $q$-ary $\overline{2}$-separable
code of length $n$ can contain at most $\frac{3}{2}q^{2\lceil
n/3\rceil}-\frac{1}{2}q^{\lceil n/3\rceil}$ codewords, and that codes with at
least $\kappa q^{2n/3}$ codewords exist. So optimal $\overline{2}$-separable
codes behave neither like $2$-frameproof nor $1$-frameproof codes.
  Also, the Gao--Ge bound is strengthened to show that a $q$-ary
$\overline{2}$-separable code of length $n$ can have at most \[ q^{\lceil
2n/3\rceil}+\tfrac{1}{2}q^{\lfloor n/3\rfloor}(q^{\lfloor n/3\rfloor}-1) \]
codewords.
