Combinatorics
Trees
Trees are yet another type of graph. Trees have all the properties of graphs except they must be connected with no cycles. A computer's hard drive directory structure is set up as a tree, with subdirectories branching out from a single root directory. Typically trees have a vertex labeled as the root vertex from which every other vertex can be reached from a unique path along the edges. Not all vertices can be a root vertex. Trees come into importance for devising searching algorithms.
Resources
Books
Berman, Gerald, and K.D. Fryer. Introduction to Combinatorics. Academic Press, 1972.
Bogard, Kenneth P. Introductory Combinatorics. Harcourt Brace Jovanovic Incorporated, 1990.
Bose R.C., and B. Manvel. Introduction to Combinatorial Theory. John Wiley & Sons, 1984.
Jackson, Bradley, and Dmitri Thoro. Applied Combinatorics with Problem Solving. Addison-Wesley, 1990.
Trudeau, Richard J. Introduction to Graph Theory. Dover, 1993.
David Gorsich
Additional topics
Science EncyclopediaScience & Philosophy: Cluster compound to ConcupiscenceCombinatorics - History Of Combinatorics, Enumeration, Binomial Coefficients, Equivalence Relations, Recurrence Relations, Graph Theory