Paper:
N.J.A. Sloane, The persistence of a number, J. ecreational Math. 6(1973), 97-98 (zip)
Mark R. Diamond, Daniel D. Reidpath, A counterexample to a conjuncture of Sloane and Erdös, J. Recreational Math. 1998 29(2), 89-92 (zip)
Sascha Kurz, Persistencen in verschiedenen Basen (german), unpublished (read critical, cause the author is still learning math ;-) ), 2001 (dvi)(ps)(pdf)(tex)