Find (for specific cases, or a general algorithm) the shortest string which contain as substrings all m digit strings made of n symbols. For example, for (2,2), the possible substrings are 00,01,10,11. One string of minimal length (which in general is n^m+m-1 digits) which contains all of these as substrings is 00110. I don't know of a general algorithim to generate the minimal superstring, or even whether it always exists. One thing I've noticed, though, is that the superstrings always seem to end with the same m-1 digits they start with, (which you can exploit to generate many more solutions), but I haven't been able to prove this.