# Jordan Normal Form - Complex Matrices

Complex Matrices

In general, a square complex matrix A is similar to a block diagonal matrix

$J = begin{bmatrix} J_1 & ; & ; \ ; & ddots & ; \ ; & ; & J_pend{bmatrix}$

where each block Ji is a square matrix of the form

$J_i = begin{bmatrix} lambda_i & 1 & ; & ; \ ; & lambda_i & ddots & ; \ ; & ; & ddots & 1 \ ; & ; & ; & lambda_i end{bmatrix}.$

So there exists an invertible matrix P such that P-1AP = J is such that the only non-zero entries of J are on the diagonal and the superdiagonal. J is called the Jordan normal form of A. Each Ji is called a Jordan block of A. In a given Jordan block, every entry on the super-diagonal is 1.

Assuming this result, we can deduce the following properties:

• Counting multiplicity, the eigenvalues of J, therefore A, are the diagonal entries.
• Given an eigenvalue λi, its geometric multiplicity is the dimension of Ker(A − λi I), and it is the number of Jordan blocks corresponding to λi.
• The sum of the sizes of all Jordan blocks corresponding to an eigenvalue λi is its algebraic multiplicity.
• A is diagonalizable if and only if, for every eigenvalue λ of A, its geometric and algebraic multiplicities coincide.
• The Jordan block corresponding to λ is of the form λ I + N, where N is a nilpotent matrix defined as Nij = δi,j−1 (where δ is the Kronecker delta). The nilpotency of N can be exploited when calculating f(A) where f is a complex analytic function. For example, in principle the Jordan form could give a closed-form expression for the exponential exp(A).