1. Gunnar S. Bali, Phys. Rep. 343, 1 (2001).
2. For the HPQCD collaboration (C.T.H. Davies), 34th International Conference on High Energy Physics, Philadelphia, 2008, arXiv:0810.3309v1.
3. J.J. Dudek, in Proceedings of the CHARM 2007 Workshop, Ithaca, NY, August 5-8, 2007, arXiv:0711.1600v1.
4. M. Gell-Mann, Phys. Lett. 8, 124 (1964).
5. M.M. Giannini, Rep. Prog. Phys. 54, 453 (1990).