Beate Bollig Martin Sauerhoff Ingo Wegener On the Non-Approximability of Boolean Functions by OBDDs and Read-K-Times Branching Programs. 172-183 http://www.computer.org/proceedings/ccc/1053/10530172abs.htm 2001 IEEE Conference on Computational Complexity db/conf/coco/coco2001.html#BolligSW01