# Generalized inverse of a symmetric matrix

This article is originally published at https://www.alexejgossmann.com

I have always found the common definition of the generalized inverse of a matrix quite unsatisfactory, because it is usually defined by a mere property, , which does not really give intuition on when such a matrix exists or on how it can be constructed, etc… But recently, I came across a much more satisfactory definition for the case of symmetric (or more general, normal) matrices. :smiley:

As is well known, any symmetric matrix is diagonalizable,

where is a diagonal matrix with the eigenvalues of on its diagonal, and is an orthogonal matrix with eigenvectors of as its columns (which magically form an orthogonal set :astonished:, just kidding, absolutely no magic involved).

### The Definition :heart:

Assume that is a real symmetric matrix of size and has rank . Denoting the *non-zero* eigenvalues of by and the corresponding columns of by , we have that

*We define the generalized inverse of* *by*

### Why this definition makes sense :triumph:

The common definition/property of generalized inverse still holds:

where we used the fact that unless (i.e., orthogonality of ).

By a similar calculation, if is invertible, then and it holds that

If is invertible, then has eigenvalues and eigenvectors (because for all ).

Thus, Definition () is simply the diagonalization of if is invertible.

Since form an orthonormal basis for the range of A, it follows that the matrix

is the projection operator onto the range of .

### But what if A is not symmetric? :fearful:

Well, then is not diagonalizable (in general), but instead we can use the singular value decomposition

and define

Easy. :relieved:

### References

Definition is mentioned in passing on page 87 in

- Morris L. Eaton,
*Multivariate Statistics: A Vector Space Approach.*Beachwood, Ohio, USA: Institute of Mathematical Statistics, 2007.

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This article is originally published at https://www.alexejgossmann.com

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