Head-on collision of ion-acoustic solitary waves in an unmagnetized electron-positron-ion plasma J. N. Han, S. C. Li, X. X. Yang and W. S. Duan Eur. Phys. J. D, 47 2 (2008) 197-201 Published online: 29 February 2008 DOI: 10.1140/epjd/e2008-00033-3