Generalized inverse


In mathematics, and in particular, algebra, a generalized inverse of an element x is an element y that has some properties of an inverse element but not necessarily all of them. Generalized inverses can be defined in any mathematical structure that involves associative multiplication, that is, in a semigroup. This article describes generalized inverses of a matrix.
Formally, given a matrix and a matrix, is a generalized inverse of if it satisfies the condition
The purpose of constructing a generalized inverse of a matrix is to obtain a matrix that can serve as an inverse in some sense for a wider class of matrices than invertible matrices. A generalized inverse exists for an arbitrary matrix, and when a matrix has a regular inverse, this inverse is its unique generalized inverse.

Motivation

Consider the linear system
where is an matrix and the column space of. If is nonsingular then will be the solution of the system. Note that, if is nonsingular, then
Now suppose is rectangular, or square and singular. Then we need a right candidate of order such that for all
That is, is a solution of the linear system.
Equivalently, we need a matrix of order such that
Hence we can define the generalized inverse or g-inverse as follows: Given an matrix, an matrix is said to be a generalized inverse of if The matrix has been termed a regular inverse of by some authors.

Types

The Penrose conditions define different generalized inverses for and
where indicates conjugate transpose. If satisfies the first condition, then it is a generalized inverse of. If it satisfies the first two conditions, then it is a reflexive generalized inverse of. If it satisfies all four conditions, then it is the pseudoinverse of. A pseudoinverse is sometimes called the Moore–Penrose inverse, after the pioneering works by E. H. Moore and Roger Penrose.
When is non-singular, any generalized inverse and is unique, but in all other cases, there are an infinite number of matrices that satisfy condition. However, the Moore–Penrose inverse is unique.
There are other kinds of generalized inverse:

Reflexive generalized inverse

Let
Since, is singular and has no regular inverse. However, and satisfy conditions and, but not or. Hence, is a reflexive generalized inverse of.

One-sided inverse

Let
Since is not square, has no regular inverse. However, is a right inverse of. The matrix has no left inverse.

Construction

The following characterizations are easy to verify:

Uses

Any generalized inverse can be used to determine whether a system of linear equations has any solutions, and if so to give all of them. If any solutions exist for the n × m linear system
with vector of unknowns and vector of constants, all solutions are given by
parametric on the arbitrary vector, where is any generalized inverse of. Solutions exist if and only if is a solution, that is, if and only if. If A has full column rank, the bracketed expression in this equation is the zero matrix and so the solution is unique.

Transformation consistency properties

In practical applications it is necessary to identify the class of matrix transformations that must be preserved by a generalized inverse. For example, the Moore-Penrose inverse, satisfies the following definition of consistency with respect to transformations involving unitary matrices U and V:
The Drazin inverse, satisfies the following definition of consistency with respect to similarity transformations involving a nonsingular matrix S:
The unit-consistent inverse, satisfies the following definition of consistency with respect to transformations involving nonsingular diagonal matrices D and E:
The fact that the Moore-Penrose inverse provides consistency with respect to rotations explains its widespread use in physics and other applications in which Euclidean distances must be preserved. The UC inverse, by contrast, is applicable when system behavior is expected to be invariant with respect to the choice of units on different state variables, e.g., miles versus kilometers.