Rant: Matrices Are Not Arrays of Numbers

From this rant:

> a matrix represents a linear map between two vector spaces

From Wikipedia (https://en.wikipedia.org/wiki/Matrix_%28mathematics%29 ) we see at least two other uses:

> A major application of matrices is to represent linear transformations, that is, generalizations of linear functions such as f(x) = 4x. ... Another application of matrices is in the solution of systems of linear equations. ... The adjacency matrix of a finite graph is a basic notion of graph theory. ...