\documentclass[12pt]{article}
\usepackage{amsmath,mathrsfs,bbm}
\usepackage{amssymb}
\textwidth=4.825in
\overfullrule=0pt
\thispagestyle{empty}
\begin{document}
\noindent
%
%
{\bf William Y.C. Chen, Oliver X.Q. Gao and Peter L. Guo}
%
%
\medskip
\noindent
%
%
{\bf On Han's Hook Length Formulas for Trees}
%
%
\vskip 5mm
\noindent
%
%
%
%
Recently, Han obtained two hook length formulas for binary trees
and asked for combinatorial proofs. One of Han's formulas
has been generalized to $k$-ary trees by Yang. Sagan has found a
probabilistic proof of Yang's extension. We give combinatorial
proofs of Yang's formula for $k$-ary trees and the other formula of
Han for binary trees. Our bijections are based on the structure of
$k$-ary trees associated with staircase labelings.
\end{document}