arhnk Hankel norm approximant

## Syntax

                slm = arhnk(sl, ord)
                slm = arhnk(sl, ord, tol)

## Arguments

sl

  linear system (`syslin` list)

ord

  integer, order of the approximant

tol

  threshold for rank determination in `equil1`

## Description

computes `slm`, the optimal Hankel norm approximant of the stable continuous-time linear system `sl` with matrices `[A,B,C,D]`.

## Examples

    A=diag([-1,-2,-3,-4,-5]);B=rand(5,1);C=rand(1,5);
    sl=syslin('c',A,B,C);
    slapprox=arhnk(sl,2);
    [nk,W]=hankelsv(sl);nk
    [nkred,Wred]=hankelsv(slapprox);nkred

## See also

- equil
- equil1
- hankelsv