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\begin{figure}\begin{center}\BoxedEPSF{Trees.epsf scaled 1000}\end{center}\end{figure}

A tree is a mathematical structure which can be viewed as either a Graph or as a Data Structure. The two views are equivalent, since a tree Data Structure contains not only a set of elements, but also connections between elements, giving a tree graph.

A tree graph is a set of straight line segments connected at their ends containing no closed loops (cycles). A tree with $n$ nodes has $n-1$ Edges. The points of connection are known as Forks and the segments as Branches. Final segments and the nodes at their ends are called Leaves. A tree with two Branches at each Fork and with one or two Leaves at the end of each branch is called a Binary Tree.

When a special node is designated to turn a tree into a Rooted Tree, it is called the Root (or sometimes ``Eve.'') In such a tree, each of the nodes which is one Edge further away from a given node is called a Child, and nodes connected to the same node which are the same distance from the Root are called Siblings.

Note that two Branches placed end-to-end are equivalent to a single Branch which means, for example, that there is only one tree of order 3. The number $t(n)$ of nonisomorphic trees of order $n=1$, 2, ... (where trees of orders 1, 2, ..., 6 are illustrated above), are 1, 1, 1, 2, 3, 6, 11, 23, 47, 106, 235, ... (Sloane's A000055).

Otter showed that

\lim_{n\to\infty} {t(n)n^{5/2}\over\alpha^n}=\beta,
\end{displaymath} (1)

(Otter 1948, Harary and Palmer 1973, Knuth 1969), where the constants $\alpha$ and $\beta$ are sometimes called Otter's Tree Enumeration Constants. Write the Generating Function for Rooted Trees as
f(z)=\sum_{i=0}^\infty f_iz^i,
\end{displaymath} (2)

where the Coefficients are
f_{i+1}={1\over i}\sum_{j=1}^i \left({\sum_{d\vert j} df_d}\right)f_{i-j+1},
\end{displaymath} (3)

with $f_0=0$ and $f_1=1$. Then
\end{displaymath} (4)

is the unique Positive Root of
f\left({1\over x}\right)=1,
\end{displaymath} (5)

\beta={1\over\sqrt{2\pi}} \left[{1+\sum_{k=2}^\infty f'\left...
...pha_k}\right){1\over\alpha_k}}\right]^{3/2} = 0.5349485\ldots.
\end{displaymath} (6)

See also B-Tree, Binary Tree, Caterpillar Graph, Cayley Tree, Child, Dijkstra Tree, Eve, Forest, Kruskal's Algorithm, Kruskal's Tree Theorem, Leaf (Tree), Orchard-Planting Problem, Ordered Tree, Path Graph, Planted Planar Tree, Pólya Enumeration Theorem, Quadtree, Red-Black Tree, Root (Tree), Rooted Tree, Sibling, Star Graph, Stern-Brocot Tree, Weakly Binary Tree, Weighted Tree


Finch, S. ``Favorite Mathematical Constants.''

Chauvin, B.; Cohen, S.; and Rouault, A. (Eds.). Trees: Workshop in Versailles, June 14-16, 1995. Basel, Switzerland: Birkhäuser, 1996.

Gardner, M. ``Trees.'' Ch. 17 in Mathematical Magic Show: More Puzzles, Games, Diversions, Illusions and Other Mathematical Sleight-of-Mind from Scientific American. New York: Vintage, pp. 240-250, 1978.

Harary, F. Graph Theory. Reading, MA: Addison-Wesley, 1994.

Harary, F. and Manvel, B. ``Trees.'' Scripta Math. 28, 327-333, 1970.

Harary, F. and Palmer, E. M. Graphical Enumeration. New York: Academic Press, 1973.

Knuth, D. E. The Art of Computer Programming, Vol. 1: Fundamental Algorithms, 2nd ed. Reading, MA: Addison-Wesley, 1973.

Otter, R. ``The Number of Trees.'' Ann. Math. 49, 583-599, 1948.

Sloane, N. J. A. Sequence A000055/M0791 in ``An On-Line Version of the Encyclopedia of Integer Sequences.'' and extended entry in Sloane, N. J. A. and Plouffe, S. The Encyclopedia of Integer Sequences. San Diego: Academic Press, 1995.

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© 1996-9 Eric W. Weisstein