Dieter Rautenbach Christian Szegedy Jürgen Werber On the cost of optimal alphabetic code trees with unequal letter costs. 386-394 2008 29 Eur. J. Comb. 2 http://dx.doi.org/10.1016/j.ejc.2007.02.014 db/journals/ejc/ejc29.html#RautenbachSW08