@article{HPV77, AUTHOR = {Hopcroft, J.E. and Paul, W.J. and Valiant L.G.},TITLE = {On Time Versus Space},VOLUME = {24}, YEAR = {1977},PAGES = {332--337}, SERIES = {J. ACM},EE = {http://doi.acm.org/10.1145/322003.322015},}