In the mathematical theory of matroids, the rank of a matroid is the maximum size of an independent set in the matroid. The rank of a subset S of elements of the matroid is, similarly, the maximum size of an independent subset of S, and the rank function of the matroid maps sets of elements to their ranks.
The rank function is one of the fundamental concepts of matroid theory via which matroids may be axiomatized. Matroid rank functions form an important subclass of the submodular set functions. The rank functions of matroids defined from certain other types of mathematical object such as undirected graphs, matrices, and field extensions are important within the study of those objects.
In all examples, E is the base set of the matroid, and B is some subset of E.
(R1) The value of the rank function is always a non-negative integer and the rank of the empty set is 0.
The rank function may be used to determine the other important properties of a matroid:
In graph theory, the circuit rank (or cyclomatic number) of a graph is the corank of the associated graphic matroid; it measures the minimum number of edges that must be removed from the graph to make the remaining edges form a forest. Several authors have studied the parameterized complexity of graph algorithms parameterized by this number.
In linear algebra, the rank of a linear matroid defined by linear independence from the columns of a matrix is the rank of the matrix, and it is also the dimension of the vector space spanned by the columns.
Matroid rank functions (MRF) has been used to represent utility functions of agents in problems of fair item allocation. If the utility function of the agent is an MRF, it means that:
The matroid-rank functions are a subclass of the gross substitute valuations.