schur
The Schur decomposition of a square matrix.
`(T, Q)=schur(a)`
 Inputs `a` Any square matrix. Outputs `u` A quasi-upper triangle matrix. `v` A unitary matrix (i.e. `Q'Q=eye(size(a))` ).

Description
The outputs of Schur satisfy `Q'*a*Q=T.` The matrix `T` is upper traiangle except that in many cases there may be non-zero elements in the sub-diagonal immediately below the main diagonal.
Example
```>>a=[1 2 3 4 ; 10 12 15 23; 15 18 23 19; 18 25 123 19]
>>a
1          2          3          4
10         12         15         23
15         18         23         19
18         25        123         19

>>[T,Q]=schur(a)
>>#Note that T is upper triangular except for the nozero element T[4,3]
>># in the lower triangular part.
>>T
85.2031     -13.3272      52.2308      -81.852
2.3093e-014      -0.5539      -5.8584      16.1396
-1.7764e-014 -6.2172e-015     -10.7226      35.6653
-1.4211e-014  7.1054e-015      -0.8419     -18.9266

>>Q
-0.0624       0.7481      -0.6027      -0.2705
-0.354      -0.6354      -0.6307      -0.2704
-0.3781       0.0849      -0.2533       0.8864
-0.8531       0.1714       0.4181      -0.2608

>>norm(Q*T*Q'-a)
2.2649e-013
```