Oscar H. Ibarra Zhe Dang Linmin Yang On Counter Machines, Reachability Problems, and Diophantine Equations. 919-934 2008 19 Int. J. Found. Comput. Sci. 4 http://dx.doi.org/10.1142/S0129054108006042 db/journals/ijfcs/ijfcs19.html#IbarraDY08