its rank, in the sense that there does not exist another element that can be added to it while maintaining the same rank.
*The difference is called the nullity of the subset . It is the minimum number of elements that must be removed from to obtain an independent set.
*The corank of a subset can refer to at least two different quantities: some authors use it to refer to the rank of in the dual matroid, , while other authors use corank to refer to the difference .

Ranks of special matroids