On the Rank of a Matrix

In this section of Resonance, we invite readers to pose questions likely to be raised in a classroom situation. We may suggest strategies for dealing with them, or invite responses, or both. “Classroom” is equally a forum for raising broader issues and sharing personal experiences and viewpoints on matters related to teaching and learning science. Two important results in linear algebra are the ‘rank-nullity theorem’ and the equality of the row and column ranks of a matrix. In this note, we will give a simple proof of the latter, using the former. As a by-product, we also prove the Fredhölm alternative, which characterizes the range of the linear operator associated with a matrix.