NAG Library Routine Document
f08gsf (zhptrd) reduces a complex Hermitian matrix to tridiagonal form, using packed storage.
|Integer, Intent (In)||:: ||n|
|Integer, Intent (Out)||:: ||info|
|Real (Kind=nag_wp), Intent (Out)||:: ||d(n), e(n-1)|
|Complex (Kind=nag_wp), Intent (Inout)||:: ||ap(*)|
|Complex (Kind=nag_wp), Intent (Out)||:: ||tau(n-1)|
|Character (1), Intent (In)||:: ||uplo|C Header Interface
f08gsf_ (const char *uplo, const Integer *n, Complex ap, double d, double e, Complex tau, Integer *info, const Charlen length_uplo)|
The routine may be called by its
f08gsf (zhptrd) reduces a complex Hermitian matrix , held in packed storage, to real symmetric tridiagonal form by a unitary similarity transformation: .
is not formed explicitly but is represented as a product of
elementary reflectors (see the F08 Chapter Introduction
for details). Routines are provided to work with
in this representation (see Section 9
Golub G H and Van Loan C F (1996) Matrix Computations (3rd Edition) Johns Hopkins University Press, Baltimore
- 1: – Character(1)Input
: indicates whether the upper or lower triangular part of
- The upper triangular part of is stored.
- The lower triangular part of is stored.
- 2: – IntegerInput
On entry: , the order of the matrix .
- 3: – Complex (Kind=nag_wp) arrayInput/Output
the dimension of the array ap
must be at least
: the upper or lower triangle of the
, packed by columns.
- if , the upper triangle of must be stored with element in for ;
- if , the lower triangle of must be stored with element in for .
is overwritten by the tridiagonal matrix
and details of the unitary matrix
- 4: – Real (Kind=nag_wp) arrayOutput
On exit: the diagonal elements of the tridiagonal matrix .
- 5: – Real (Kind=nag_wp) arrayOutput
On exit: the off-diagonal elements of the tridiagonal matrix .
- 6: – Complex (Kind=nag_wp) arrayOutput
On exit: further details of the unitary matrix .
- 7: – IntegerOutput
unless the routine detects an error (see Section 6
Error Indicators and Warnings
If , argument had an illegal value. An explanatory message is output, and execution of the program is terminated.
The computed tridiagonal matrix
is exactly similar to a nearby matrix
is a modestly increasing function of
is the machine precision
The elements of themselves may be sensitive to small perturbations in or to rounding errors in the computation, but this does not affect the stability of the eigenvalues and eigenvectors.
Parallelism and Performance
f08gsf (zhptrd) makes calls to BLAS and/or LAPACK routines, which may be threaded within the vendor library used by this implementation. Consult the documentation for the vendor library for further information.
Please consult the X06 Chapter Introduction
for information on how to control and interrogate the OpenMP environment used within this routine. Please also consult the Users' Note
for your implementation for any additional implementation-specific information.
The total number of real floating-point operations is approximately .
To form the unitary matrix f08gsf (zhptrd)
may be followed by a call to f08gtf (zupgtr)
complex matrix f08gsf (zhptrd)
may be followed by a call to f08guf (zupmtr)
. For example,
Call zupmtr('Left',uplo,'No Transpose',n,p,ap,tau,c,ldc,work, &
forms the matrix product
The real analogue of this routine is f08gef (dsptrd)
This example reduces the matrix
to tridiagonal form, where
using packed storage.
Program Text (f08gsfe.f90)
Program Data (f08gsfe.d)
Program Results (f08gsfe.r)