Tree is the term from the branch of the mathematics which known as the graph theory, used to describe any set of the straight-line segments joining the pairs of points in some probably of the multidimensional space, so that

* each point is linked to every other point by the set of lines,

* every point is visited by at least one of the lines,

* no closed circuits seems in the structure.

If length of any of the segment is given by the distance between two points it connects, then the length of the tree is defined to be the sum of the lengths of its segments. Particularly significant in some aspects of the multivariate analysis is the minimum spanning tree of the set of n points which is simply tree of shortest length among all trees which could be constructed from these points.