NAG Toolbox: nag_lapack_zunmql (f08cu)

Purpose

Syntax

Description

References

Parameters

Compulsory Input Parameters

1:     $\mathrm{side}$ – string (length ≥ 1)

Indicates how $Q$ or $Q^{\mathrm{H}}$ is to be applied to $C$.

$\mathbf{side}=\text{'L'}$

$Q$ or $Q^{\mathrm{H}}$ is applied to $C$ from the left.

$\mathbf{side}=\text{'R'}$

$Q$ or $Q^{\mathrm{H}}$ is applied to $C$ from the right.

Constraint: $\mathbf{side}=\text{'L'}$ or $\text{'R'}$.

2:     $\mathrm{trans}$ – string (length ≥ 1)

Indicates whether $Q$ or $Q^{\mathrm{H}}$ is to be applied to $C$.

$\mathbf{trans}=\text{'N'}$

$Q$ is applied to $C$.

$\mathbf{trans}=\text{'C'}$

$Q^{\mathrm{H}}$ is applied to $C$.

Constraint: $\mathbf{trans}=\text{'N'}$ or $\text{'C'}$.

3:     $\mathrm{a}\left(\mathit{lda},:\right)$ – complex array

The first dimension, $\mathit{lda}$, of the array a must satisfy

• if $\mathbf{side}=\text{'L'}$, $\mathit{lda}\ge \max \left(1,\mathbf{m}\right)$;
• if $\mathbf{side}=\text{'R'}$, $\mathit{lda}\ge \max \left(1,\mathbf{n}\right)$.

The second dimension of the array a must be at least $\max \left(1,\mathbf{k}\right)$.

Details of the vectors which define the elementary reflectors, as returned by nag_lapack_zgeqlf (f08cs).

4:     $\mathrm{tau}\left(:\right)$ – complex array

The dimension of the array tau must be at least $\max \left(1,\mathbf{k}\right)$

Further details of the elementary reflectors, as returned by nag_lapack_zgeqlf (f08cs).

5:     $\mathrm{c}\left(\mathit{ldc},:\right)$ – complex array

The first dimension of the array c must be at least $\max \left(1,\mathbf{m}\right)$.

The second dimension of the array c must be at least $\max \left(1,\mathbf{n}\right)$.

The $m$ by $n$ matrix $C$.

Optional Input Parameters

1:     $\mathrm{m}$ – int64int32nag_int scalar

Default: the first dimension of the array c.

$m$, the number of rows of the matrix $C$.

Constraint: $\mathbf{m}\ge 0$.

2:     $\mathrm{n}$ – int64int32nag_int scalar

Default: the second dimension of the array c.

$n$, the number of columns of the matrix $C$.

Constraint: $\mathbf{n}\ge 0$.

3:     $\mathrm{k}$ – int64int32nag_int scalar

Default: the second dimension of the arrays a, tau.

$k$, the number of elementary reflectors whose product defines the matrix $Q$.

Constraints:

• if $\mathbf{side}=\text{'L'}$, $\mathbf{m}\ge \mathbf{k}\ge 0$;
• if $\mathbf{side}=\text{'R'}$, $\mathbf{n}\ge \mathbf{k}\ge 0$.

Output Parameters

1:     $\mathrm{c}\left(\mathit{ldc},:\right)$ – complex array

The first dimension of the array c will be $\max \left(1,\mathbf{m}\right)$.

The second dimension of the array c will be $\max \left(1,\mathbf{n}\right)$.

c stores $QC$ or $Q^{\mathrm{H}}C$ or $CQ$ or $C{Q}^{\mathrm{H}}$ as specified by side and trans.

2:     $\mathrm{info}$ – int64int32nag_int scalar

$\mathbf{info}=0$ unless the function detects an error (see Error Indicators and Warnings).

Error Indicators and Warnings

   $\mathbf{info}=-i$

If $\mathbf{info}=-i$, parameter $i$ had an illegal value on entry. The parameters are numbered as follows:

1: side, 2: trans, 3: m, 4: n, 5: k, 6: a, 7: lda, 8: tau, 9: c, 10: ldc, 11: work, 12: lwork, 13: info.

It is possible that info refers to a parameter that is omitted from the MATLAB interface. This usually indicates that an error in one of the other input parameters has caused an incorrect value to be inferred.

Accuracy

Further Comments

Example

Open in the MATLAB editor: f08cu_example

function f08cu_example


fprintf('f08cu example results\n\n');

m = 6;
n = 4;
a = [ 0.96 - 0.81i, -0.03 + 0.96i, -0.91 + 2.06i, -0.05 + 0.41i;
     -0.98 + 1.98i, -1.20 + 0.19i, -0.66 + 0.42i, -0.81 + 0.56i;
      0.62 - 0.46i,  1.01 + 0.02i,  0.63 - 0.17i, -1.11 + 0.60i;
     -0.37 + 0.38i,  0.19 - 0.54i, -0.98 - 0.36i,  0.22 - 0.20i;
      0.83 + 0.51i,  0.20 + 0.01i, -0.17 - 0.46i,  1.47 + 1.59i;
      1.08 - 0.28i,  0.20 - 0.12i, -0.07 + 1.23i,  0.26 + 0.26i];

b = [-2.09 + 1.93i,  3.26 - 2.70i;
      3.34 - 3.53i, -6.22 + 1.16i;
     -4.94 - 2.04i,  7.94 - 3.13i;
      0.17 + 4.23i,  1.04 - 4.26i;
     -5.19 + 3.63i, -2.31 - 2.12i;
      0.98 + 2.53i, -1.39 - 4.05i];

% Compute the QL factorization of A
[ql, tau, info] = f08cs(a);

% LX = Q^H B = C; compute C = (Q^H)*B
[c, info] = f08cu( ...
		   'Left', 'ConjTrans', ql, tau, b);

% Least-squares solution X = L^-1 C (lower n part)
il = m-n+1;
[x, info] = f07ts( ...
		   'Lower', 'Notrans','Non-Unit', ql(il:m,:), c(il:m,:));

% Print least-squares solutions
ncols  = int64(80);
indent = int64(0);
[ifail] = x04db( ...
		 'General', ' ', x, 'Bracketed', 'F7.4', ...
		 'Least-squares solution(s)', 'Integer', 'Integer', ...
		 ncols, indent);

% Compute estimates of the square roots of the residual sums of squares.
rnorm = zeros(2,1);
for j=1:2
  rnorm(j) = norm(c(1:m-n,j));
end
fprintf('\nSquare root(s) of the residual sum(s) of squares\n');
fprintf('\t%11.2e    %11.2e\n', rnorm(1), rnorm(2));

f08cu example results

 Least-squares solution(s)
                    1                 2
 1  (-0.5044,-1.2179) ( 0.7629, 1.4529)
 2  (-2.4281, 2.8574) ( 5.1570,-3.6089)
 3  ( 1.4872,-2.1955) (-2.6518, 2.1203)
 4  ( 0.4537, 2.6904) (-2.7606, 0.3318)

Square root(s) of the residual sum(s) of squares
	   6.88e-02       1.87e-01