We introduce a new -as far as we know- problem, according to which we are asked to match sequences of two digits in matrices having entries among those two digits (but others too) and prove that this problem is NP-complete
@article{arxiv.1104.2083,
title = {Matching sequences of two digits in matrices is hard},
author = {Nicolaos Matsakis},
journal= {arXiv preprint arXiv:1104.2083},
year = {2011}
}