unobs unobservable subspace

## Syntax

                [n, U] = unobs(A, C)
                [n, U] = unobs(A, C, tol)

## Arguments

A, C

  real matrices

tol

  tolerance used when evaluating ranks (QR factorizations).

n

  dimension of unobservable subspace.

U

  orthogonal change of basis which puts `(A,B)` in canonical form.

## Description

`[n,[U]]=unobs(A,C,[tol])` gives the unobservable form of an `(A,C)` pair. The `n` first columns of `U` make a basis for the unobservable subspace.

The `(2,1)` block (made of last `nx-n` rows and `n` first columns) of `U'*A*U` is zero and the `n` first columns of `C*U` are zero.

## Examples

    A=diag([1,2,3]);C=[1,0,0];
    unobs(A,C)

## See also

- contr
- contrss
- canon
- cont_mat
- spantwo
- dt_ility