# Multitree

In combinatorics and order-theoretic mathematics, a multitree may describe either of two equivalent structures: a directed acyclic graph (DAG) in which the set of vertices reachable from any vertex induces a tree, or a partially ordered set (poset) that does not have four items a, b, c, and d forming a diamond suborder with abd and acd but with b and c incomparable to each other (also called a diamond-free poset[1]).

In computational complexity theory, multitrees have also been called strongly unambiguous graphs or mangroves; they can be used to model nondeterministic algorithms in which there is at most one computational path connecting any two states.[2]

Multitrees may be used to represent multiple overlapping taxonomies over the same ground set.[3] If a family tree may contain multiple marriages from one family to another, but does not contain marriages between any two blood relatives, then it forms a multitree.[4]

## Equivalence between DAG and poset definitions

In a directed acyclic graph, if the set of vertices reachable from any vertex induces a tree, or equivalently if there is at most one directed path between any two vertices in either direction, then its reachability relation is a diamond-free partial order. Conversely, in a partial order, if it is diamond-free, then its transitive reduction identify a directed acyclic graph in which the set of vertices reachable from any vertex induces a tree

## Diamond-free families

A diamond-free family of sets is a family F of sets whose inclusion ordering forms a diamond-free poset. If D(n) denotes the largest possible diamond-free family of subsets of an n-element set, then it is known that

${\displaystyle 2\leq \lim _{n\to \infty }D(n){\Big /}{\binom {n}{\lfloor n/2\rfloor }}\leq 2{\frac {3}{11}}}$

and it is conjectured that the limit is 2.[1]

A polytree, a directed acyclic graph formed by assigning an orientation to each edge of an undirected tree, may be viewed as a special case of a multitree.

The set of all vertices connected to any vertex in a multitree forms an arborescence.

The word "multitree" has also been used to refer to a series-parallel partial order,[5] or to other structures formed by combining multiple trees.

## References

1. Griggs, Jerrold R.; Li, Wei-Tian; Lu, Linyuan (2010), Diamond-free families, arXiv:1010.5311, Bibcode:2010arXiv1010.5311G.
2. Allender, Eric; Lange, Klaus-Jörn (1996), "StUSPACE(log n) ⊆ DSPACE(log2 n/log log n)", Algorithms and Computation, 7th International Symposium, ISAAC '96, Osaka, Japan, December 16–18, 1996, Proceedings, Lecture Notes in Computer Science, 1178, Springer-Verlag, pp. 193–202, doi:10.1007/BFb0009495.
3. Furnas, George W.; Zacks, Jeff (1994), "Multitrees: enriching and reusing hierarchical structure", Proc. SIGCHI conference on Human Factors in Computing Systems (CHI '94), pp. 330–336, doi:10.1145/191666.191778.
4. McGuffin, Michael J.; Balakrishnan, Ravin (2005), "Interactive visualization of genealogical graphs", IEEE Symposium on Information Visualization, Los Alamitos, CA, USA: IEEE Computer Society, pp. 3–3, doi:10.1109/INFOVIS.2005.22.
5. Jung, H. A. (1978), "On a class of posets and the corresponding comparability graphs", Journal of Combinatorial Theory, Series B, 24 (2): 125–133, doi:10.1016/0095-8956(78)90013-8, MR 0491356.