The external path length of an extended binary tree is the sum over all external (square) nodes of the lengths of the paths from the root to each node. For example, in the tree above, the external path length is 25 (Knuth 1997, pp. 399-400). The internal and external path lengths are related by
where 
is the number of internal nodes.
See also
Extended Binary Tree, Internal Path Length
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References
Knuth, D. E. The Art of Computer Programming, Vol. 1: Fundamental Algorithms, 3rd ed. Reading, MA: Addison-Wesley, 1997.
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Cite this as:
Weisstein, Eric W. "External Path Length." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/ExternalPathLength.html