NAG Library Routine Document

F08HEF (DSBTRD)

1  Purpose

2  Specification

3  Description

4  References

5  Parameters

1:     VECT – CHARACTER(1)Input

On entry: indicates whether $Q$ is to be returned.

$\mathbf{VECT}=\text{'V'}$

$Q$ is returned.

$\mathbf{VECT}=\text{'U'}$

$Q$ is updated (and the array Q must contain a matrix on entry).

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

$Q$ is not required.

Constraint: $\mathbf{VECT}=\text{'V'}$, $\text{'U'}$ or $\text{'N'}$.

2:     UPLO – CHARACTER(1)Input

On entry: indicates whether the upper or lower triangular part of $A$ is stored.

$\mathbf{UPLO}=\text{'U'}$

The upper triangular part of $A$ is stored.

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

The lower triangular part of $A$ is stored.

Constraint: $\mathbf{UPLO}=\text{'U'}$ or $\text{'L'}$.

3:     N – INTEGERInput

On entry: $n$, the order of the matrix $A$.

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

4:     KD – INTEGERInput

On entry: if $\mathbf{UPLO}=\text{'U'}$, the number of superdiagonals, $k_d$, of the matrix $A$.

If $\mathbf{UPLO}=\text{'L'}$, the number of subdiagonals, $k_d$, of the matrix $A$.

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

5:     AB(LDAB,$*$) – REAL (KIND=nag_wp) arrayInput/Output

Note: the second dimension of the array AB must be at least $\max \left(1,\mathbf{N}\right)$.

On entry: the upper or lower triangle of the $n$ by $n$ symmetric band matrix $A$.

The matrix is stored in rows $1$ to $k_d+1$, more precisely,

• if $\mathbf{UPLO}=\text{'U'}$, the elements of the upper triangle of $A$ within the band must be stored with element $A_{ij}$ in $\mathbf{AB}\left(k_d+1+i-j,j\right)\text{ for }\max \left(1,j-k_d\right)\le i\le j$;
• if $\mathbf{UPLO}=\text{'L'}$, the elements of the lower triangle of $A$ within the band must be stored with element $A_{ij}$ in $\mathbf{AB}\left(1+i-j,j\right)\text{ for }j\le i\le \min \left(n,j+k_d\right)\text{.}$

On exit: AB is overwritten by values generated during the reduction to tridiagonal form.

The first superdiagonal or subdiagonal and the diagonal of the tridiagonal matrix $T$ are returned in AB using the same storage format as described above.

6:     LDAB – INTEGERInput

On entry: the first dimension of the array AB as declared in the (sub)program from which F08HEF (DSBTRD) is called.

Constraint: $\mathbf{LDAB}\ge \max \left(1,\mathbf{KD}+1\right)$.

7:     D(N) – REAL (KIND=nag_wp) arrayOutput

On exit: the diagonal elements of the tridiagonal matrix $T$.

8:     E($\mathbf{N}-1$) – REAL (KIND=nag_wp) arrayOutput

On exit: the off-diagonal elements of the tridiagonal matrix $T$.

9:     Q(LDQ,$*$) – REAL (KIND=nag_wp) arrayInput/Output

Note: the second dimension of the array Q must be at least $\max \left(1,\mathbf{N}\right)$ if $\mathbf{VECT}=\text{'V'}$ or $\text{'U'}$ and at least $1$ if $\mathbf{VECT}=\text{'N'}$.

On entry: if $\mathbf{VECT}=\text{'U'}$, Q must contain the matrix formed in a previous stage of the reduction (for example, the reduction of a banded symmetric-definite generalized eigenproblem); otherwise Q need not be set.

On exit: if $\mathbf{VECT}=\text{'V'}$ or $\text{'U'}$, the $n$ by $n$ matrix $Q$.

If $\mathbf{VECT}=\text{'N'}$, Q is not referenced.

10:   LDQ – INTEGERInput

On entry: the first dimension of the array Q as declared in the (sub)program from which F08HEF (DSBTRD) is called.

Constraints:

• if $\mathbf{VECT}=\text{'V'}$ or $\text{'U'}$, $\mathbf{LDQ}\ge \max \left(1,\mathbf{N}\right)$;
• if $\mathbf{VECT}=\text{'N'}$, $\mathbf{LDQ}\ge 1$.

11:   WORK(N) – REAL (KIND=nag_wp) arrayWorkspace

12:   INFO – INTEGEROutput

On exit: $\mathbf{INFO}=0$ unless the routine detects an error (see Section 6).

6  Error Indicators and Warnings

$\mathbf{INFO}<0$

If $\mathbf{INFO}=-i$, argument $i$ had an illegal value. An explanatory message is output, and execution of the program is terminated.

7  Accuracy

8  Further Comments

9  Example

9.1  Program Text

Program Text (f08hefe.f90)

9.2  Program Data

Program Data (f08hefe.d)

9.3  Program Results

Program Results (f08hefe.r)