more challenge sequences needed

[N. J. A. Sloane]

To Seqfans, Mathfun:  the people who are planning the contest
(I should probably not have described them as French, although
my contact has a French email address - they could be
anywhere in the world) replied that none of the sequences
that I gave them were suitable.  Either they failed to meet
some of the requirements for a good challenge, or else
they were too close to problems that had been used
in previous challenges.

So they are still looking for a good sequence to extend.
Something like the Hadamard determinant problem or the
Golomb ruler problem would be perfect, but these have
already been used.

Here are the constraints on the problem:

1) It must be NP-complete
2) It must be simple to describe
3) It should not have been widely explored
4) A solution must be easy to check (= it doesn't require a lot of
computation to check)
5) It has to be computed from N=8 to at most 64


Further suggestions would be welcomed!

Neil