hankelsv Hankel singular values

## Syntax

                [nk2, W] = hankelsv(sl)
                [nk2, W] = hankelsv(sl, tol)

## Arguments

sl

  `syslin` list representing the linear system (state-space).

tol

  tolerance parameter for detecting imaginary axis modes (default value is `1000*%eps`).

## Description

returns `nk2`, the squared Hankel singular values of `sl` and `W = P*Q` = controllability gramian times observability gramian.

`nk2` is the vector of eigenvalues of `W`.

## Examples

    A=diag([-1,-2,-3]);
    sl=syslin('c',A,rand(3,2),rand(2,3));[nk2,W]=hankelsv(sl)
    [Q,M]=pbig(W,nk2(2)-%eps,'c');
    slr=projsl(sl,Q,M);hankelsv(slr)

## See also

- balreal
- equil
- equil1