David S. Herscovici Aparna W. Higgins The pebbling number of C<sub>5</sub> × C<sub>5</sub>. 123-135 1998 187 Discrete Mathematics 1-3 http://dx.doi.org/10.1016/S0012-365X(97)00229-X db/journals/dm/dm187.html#HerscoviciH98