lyap Lyapunov equation

## Syntax

    [X]=lyap(A,C,'c')
                [X]=lyap(A,C,'d')

## Arguments

A, C

  real square matrices, `C` must be symmetric

## Description

`X= lyap(A,C,flag)` solves the continuous time or discrete time matrix Lyapunov matrix equation:

    A'*X + X*A = C       ( flag='c' )
    A'*X*A - X = C       ( flag='d' )

Note that a unique solution exist if and only if an eigenvalue of `A` is not an eigenvalue of `-A` (`flag='c'`) or 1 over an eigenvalue of `A` (`flag='d'`).

## Examples

    A=rand(4,4);C=rand(A);C=C+C';
    X=lyap(A,C,'c');
    A'*X + X*A -C
    X=lyap(A,C,'d');
    A'*X*A - X -C

## See also

- sylv
- ctr_gram
- obs_gram