Christian Choffrut Flavio D'Alessandro Stefano Varricchio On the separability of sparse context-free languages and of bounded rational relations. 274-279 2007 381 Theor. Comput. Sci. 1-3 http://dx.doi.org/10.1016/j.tcs.2007.04.003 db/journals/tcs/tcs381.html#ChoffrutDV07