A rectangular array of mathematical expressions is called a matrix. A matrix with m rows and n columns is referred to as an m×n matrix. Matrices are sometimes referred to simply as arrays, and an m×1 or 1×n array is also called a vector .
Entries in matrices can be real or complex numbers, or mathematical expressions with real or complex coefficients. Most of the choices from the Matrices submenu will operate on both real and complex matrices. The QR and SVD factorizations discussed later in this chapter assume real matrices.
Matrix entries are identified by their row and column number. The matrix can be considered as a function on pairs of positive integers and, if the matrix is given a name, this feature can be used to retrieve the entries.