# qz

Generalized Schur (QZ) factorization for generalized eigenvalues

## Description

## Examples

### QZ Factorization

Calculate the QZ factorization of two 3-by-3 matrices.

A = [1 7 3; 2 9 12; 5 22 7]; B = [3 1 0; 0 3 1; 0 0 3]; [AA,BB,Q,Z] = qz(A,B)

`AA = `*3×3*
23.5574 1.4134 -14.3485
0 -0.5776 2.7629
0 0 -8.6720

`BB = `*3×3*
3.5845 -0.1090 -0.6024
0 2.7599 0.8430
0 0 2.7292

`Q = `*3×3*
0.2566 0.6353 0.7284
-0.9477 0.3134 0.0604
-0.1899 -0.7058 0.6824

`Z = `*3×3*
0.1502 -0.9664 -0.2088
0.4689 0.2556 -0.8455
0.8704 0.0291 0.4915

Verify that the norms of `AA - Q*A*Z`

, `BB - Q*B*Z`

, `Q'*Q - eye(size(Q))`

, and `Z'*Z - eye(size(Z))`

are 0, within machine precision.

norm(AA - Q*A*Z)

ans = 6.3184e-15

norm(BB - Q*B*Z)

ans = 1.7936e-15

norm(Q'*Q - eye(size(Q)))

ans = 8.8320e-16

norm(Z'*Z - eye(size(Z)))

ans = 5.1071e-16

### Generalized Eigenvectors

Calculate the QZ factorization and also return the generalized eigenvectors of two 2-by-2 matrices.

A = [10 -7; -3 2]; B = [7 3; 12 9]; [AA,BB,Q,Z,V,W] = qz(A,B)

`AA = `*2×2*
11.9600 -4.3532
0 -0.0836

`BB = `*2×2*
1.6381 -2.9374
0 16.4830

`Q = `*2×2*
-0.9597 0.2811
0.2811 0.9597

`Z = `*2×2*
-0.5752 0.8180
0.8180 0.5752

`V = `*2×2*
-0.7031 0.6960
1.0000 1.0000

`W = `*2×2*
-1.0000 0.2929
0.4537 1.0000

Verify that the elements of `Q*A*Z - AA`

and `Q*B*Z - BB`

are 0, within machine precision.

Q*A*Z - AA

ans =2×210^{-14}× 0 0.1776 -0.1034 -0.1360

Q*B*Z - BB

ans =2×210^{-14}× -0.0222 0 0.0888 -0.3553

Calculate the generalized eigenvalues and right and left eigenvectors of `A`

and `B`

by using the `eig`

function. Verify that the elements of `A*V - B*V*D`

and `W'*A - D*W'*B`

are 0, within machine precision.

[V,D,W] = eig(A,B); A*V - B*V*D

ans =2×210^{-14}× 0 -0.2297 -0.7105 -0.0860

W'*A - D*W'*B

ans =2×210^{-14}× 0.7105 -0.3553 -0.1235 -0.0625

### Complex QZ Factorization

Calculate the complex QZ factorization of two 3-by-3 matrices.

A = [1/sqrt(2) 1 0; 0 1 1; 0 1/sqrt(2) 1]; B = [0 1 1; -1/sqrt(2) 0 1; 1 -1/sqrt(2) 0]; [AAc,BBc,Qc,Zc] = qz(A,B)

`AAc = `*3×3 complex*
0.5011 - 0.8679i 0.0332 - 1.0852i 0.3687 + 0.9278i
0.0000 + 0.0000i 0.1848 - 0.0000i -0.6334 - 0.3673i
0.0000 + 0.0000i 0.0000 + 0.0000i 0.5590 + 0.9682i

`BBc = `*3×3 complex*
1.0022 + 0.0000i 0.3136 + 0.0711i -0.0280 + 0.5966i
0.0000 + 0.0000i 1.3388 + 0.0000i 0.1572 + 0.6846i
0.0000 + 0.0000i 0.0000 + 0.0000i 1.1180 + 0.0000i

`Qc = `*3×3 complex*
0.5379 + 0.2210i 0.4604 - 0.3553i 0.3214 - 0.4693i
0.2172 + 0.3386i 0.4018 - 0.0188i -0.7698 + 0.2895i
-0.3719 - 0.6014i 0.7068 - 0.0213i -0.0000 - 0.0000i

`Zc = `*3×3 complex*
0.2514 + 0.0413i -0.7279 - 0.4531i -0.4470 - 0.0135i
-0.1000 - 0.6068i 0.3328 - 0.3332i -0.3326 + 0.5379i
0.6391 + 0.3853i 0.1423 - 0.1511i 0.2996 + 0.5570i

Calculate the real QZ decomposition of `A`

and `B`

by specifying `mode`

as `"real`

". The generalized Schur form of `A`

is quasitriangular, indicating that it has complex eigenvalues.

`[AAr,BBr,Qr,Zr] = qz(A,B,"real")`

`AAr = `*3×3*
0.1464 1.1759 0.3094
0 1.0360 1.2594
0 -0.8587 0.3212

`BBr = `*3×3*
1.0607 -0.5952 0.1441
0 1.6676 0
0 0 0.8481

`Qr = `*3×3*
0.0000 -0.0000 -1.0000
-0.7882 -0.6154 0.0000
-0.6154 0.7882 -0.0000

`Zr = `*3×3*
-0.7071 0.2610 -0.6572
0.5000 -0.4727 -0.7257
-0.5000 -0.8417 0.2037

For triangular `AAc`

, compute the eigenvalues by using `diag(AA)./diag(BB)`

.

diag(AAc)./diag(BBc)

`ans = `*3×1 complex*
0.5000 - 0.8660i
0.1381 - 0.0000i
0.5000 + 0.8660i

For quasitriangular `AAr`

, compute the eigenvalues by using the `ordeig`

function.

ordeig(AAr,BBr)

`ans = `*3×1 complex*
0.1381 + 0.0000i
0.5000 + 0.8660i
0.5000 - 0.8660i

## Input Arguments

`A`

, `B`

— Input matrices

square matrices

Input matrices, specified as real or complex square matrices. The dimensions of
`A`

and `B`

must be the same.

**Data Types: **`single`

| `double`

**Complex Number Support: **Yes

`mode`

— Decomposition mode

`"complex"`

(default) | `"real"`

Decomposition mode, specified as one of these values:

`"complex"`

—`qz`

returns a possibly complex decomposition, and`AA`

and`BB`

are triangular.`"real"`

—`qz`

returns a real decomposition, and`AA`

and`BB`

are quasitriangular.

## Output Arguments

`AA`

, `BB`

— Generalized Schur forms of `A`

and `B`

square matrices

Generalized Schur forms of `A`

and `B`

,
returned as upper triangular or quasitriangular square matrices.

When the decomposition is complex and

`AA`

is triangular, then the diagonal elements`a = diag(AA)`

and`b = diag(BB)`

are the generalized eigenvalues that satisfy`A*V*b = B*V*a`

and`b'*W'*A = a'*W'*B`

.When the decomposition is real and

`AA`

is quasitriangular, you must further reduce the 2-by-2 blocks to obtain the eigenvalues of the full system. Each 2-by-2 block in`AA`

corresponds to a 2-by-2 diagonal block at the same location in`BB`

.

`Q`

, `Z`

— Unitary factors

square matrices

Unitary factors, returned as square matrices that satisfy ```
Q*A*Z =
AA
```

and `Q*B*Z = BB`

.

`V`

— Right eigenvectors

square matrix

Right eigenvectors, returned as a square matrix whose columns are the generalized
right eigenvectors of the pair `(A,B)`

. The eigenvectors satisfy
`A*V = B*V*D`

, where `D`

contains the generalized
eigenvalues of the pair along its main diagonal. Use the `eig`

function to return `D`

and the `ordeig`

function to return the diagonal elements of
`D`

.

Different machines and releases of MATLAB^{®} can produce different eigenvectors that are still numerically accurate:

For real eigenvectors, the sign of the eigenvectors can change.

For complex eigenvectors, the eigenvectors can be multiplied by any complex number of magnitude 1.

For a multiple eigenvalue, its eigenvectors can be recombined through linear combinations. For example, if

*A**x*=*λ**x*and*A**y*=*λ**y*, then*A*(*x*+*y*) =*λ*(*x*+*y*), so*x*+*y*also is an eigenvector of*A*.

`W`

— Left eigenvectors

square matrix

Left eigenvectors, returned as a square matrix whose columns are the generalized
left eigenvectors of the pair `(A,B)`

. The eigenvectors satisfy
`W'*A = D*W'*B`

, where `D`

contains the generalized
eigenvalues of the pair along its main diagonal. Use the `eig`

function to return `D`

and the `ordeig`

function to
return the diagonal elements of `D`

.

Different machines and releases of MATLAB can produce different eigenvectors that are still numerically accurate:

For real eigenvectors, the sign of the eigenvectors can change.

For complex eigenvectors, the eigenvectors can be multiplied by any complex number of magnitude 1.

For a multiple eigenvalue, its eigenvectors can be recombined through linear combinations. For example, if

*A**x*=*λ**x*and*A**y*=*λ**y*, then*A*(*x*+*y*) =*λ*(*x*+*y*), so*x*+*y*also is an eigenvector of*A*.

## More About

### Quasitriangular Matrix

An upper quasitriangular matrix can result from the Schur decomposition or generalized Schur (QZ) decomposition of a real matrix. An upper quasitriangular matrix is block upper triangular, with 1-by-1 and 2-by-2 blocks of nonzero values along the diagonal.

The eigenvalues of these diagonal blocks are also the eigenvalues of the matrix. The 1-by-1 blocks correspond to real eigenvalues, and the 2-by-2 blocks correspond to complex conjugate eigenvalue pairs.

### Unitary Matrix

An invertible complex square matrix `U`

is unitary if its conjugate
transpose is also its inverse, that is, if $${U}^{*}U=U{U}^{*}=I$$.

## Tips

You can calculate the generalized eigenvalues that solve the generalized eigenvalue problem $$Ax=\lambda Bx$$ from the QZ factorization. For triangular

`AA`

, calculate the eigenvalues using`diag(AA)./diag(BB)`

. For quasitriangular`AA`

, calculate the eigenvalues using`ordeig(AA,BB)`

.

## Extended Capabilities

### Thread-Based Environment

Run code in the background using MATLAB® `backgroundPool`

or accelerate code with Parallel Computing Toolbox™ `ThreadPool`

.

This function fully supports thread-based environments. For more information, see Run MATLAB Functions in Thread-Based Environment.

## Version History

**Introduced before R2006a**

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