Label the vertices of a tree (with $n$ vertices) with integers from $1$ to $n$. Now label each edge with the absolute difference of the labels of its incident vertices. The labeling is said to be graceful if the edges are labeled $1$ through $n-1$ inclusive (with no number repeated). A tree is called graceful if it has at least one such labeling.
Posted: Jun 26 '12
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Last updated: Jun 26 '12
