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{\bf William Y.C. Chen, Oliver X.Q. Gao and Peter  L. Guo}
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{\bf On Han's Hook Length Formulas for Trees}
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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.

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