Andrzej Czygrinow Glenn Hurlbert Girth, Pebbling, and Grid Thresholds. 1-10 2006 20 SIAM J. Discrete Math. 1 http://epubs.siam.org/SIDMA/volume-20/art_41637.html db/journals/siamdm/siamdm20.html#CzygrinowH06