Cor A. J. Hurkens Leo van Iersel Judith Keijsper Steven Kelk Leen Stougie John Tromp Prefix Reversals on Binary and Ternary Strings. 592-611 2007 21 SIAM J. Discrete Math. 3 http://dx.doi.org/10.1137/060664252 db/journals/siamdm/siamdm21.html#HurkensIKKST07