NAG FL Interface
f07wdf (dpftrf)

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1 Purpose

f07wdf computes the Cholesky factorization of a real symmetric positive definite matrix stored in Rectangular Full Packed (RFP) format.

2 Specification

Fortran Interface
Subroutine f07wdf ( transr, uplo, n, ar, info)
Integer, Intent (In) :: n
Integer, Intent (Out) :: info
Real (Kind=nag_wp), Intent (Inout) :: ar(n*(n+1)/2)
Character (1), Intent (In) :: transr, uplo
C Header Interface
#include <nag.h>
void  f07wdf_ (const char *transr, const char *uplo, const Integer *n, double ar[], Integer *info, const Charlen length_transr, const Charlen length_uplo)
The routine may be called by the names f07wdf, nagf_lapacklin_dpftrf or its LAPACK name dpftrf.

3 Description

f07wdf forms the Cholesky factorization of a real symmetric positive definite matrix A either as A=UTU if uplo='U' or A=LLT if uplo='L', where U is an upper triangular matrix and L is a lower triangular, stored in RFP format. The RFP storage format is described in Section 3.3.3 in the F07 Chapter Introduction.

4 References

Demmel J W (1989) On floating-point errors in Cholesky LAPACK Working Note No. 14 University of Tennessee, Knoxville https://www.netlib.org/lapack/lawnspdf/lawn14.pdf
Gustavson F G, Waśniewski J, Dongarra J J and Langou J (2010) Rectangular full packed format for Cholesky's algorithm: factorization, solution, and inversion ACM Trans. Math. Software 37, 2

5 Arguments

1: transr Character(1) Input
On entry: specifies whether the RFP representation of A is normal or transposed.
transr='N'
The matrix A is stored in normal RFP format.
transr='T'
The matrix A is stored in transposed RFP format.
Constraint: transr='N' or 'T'.
2: uplo Character(1) Input
On entry: specifies whether the upper or lower triangular part of A is stored.
uplo='U'
The upper triangular part of A is stored, and A is factorized as UTU, where U is upper triangular.
uplo='L'
The lower triangular part of A is stored, and A is factorized as LLT, where L is lower triangular.
Constraint: uplo='U' or 'L'.
3: n Integer Input
On entry: n, the order of the matrix A.
Constraint: n0.
4: ar(n×(n+1)/2) Real (Kind=nag_wp) array Input/Output
On entry: the upper or lower triangular part (as specified by uplo) of the n×n symmetric matrix A, in either normal or transposed RFP format (as specified by transr). The storage format is described in detail in Section 3.3.3 in the F07 Chapter Introduction.
On exit: if info=0, the factor U or L from the Cholesky factorization A=UTU or A=LLT, in the same storage format as A.
5: info Integer Output
On exit: info=0 unless the routine detects an error (see Section 6).

6 Error Indicators and Warnings

info<0
If info=-i, argument i had an illegal value. An explanatory message is output, and execution of the program is terminated.
info>0
The leading minor of order value is not positive definite and the factorization could not be completed. Hence A itself is not positive definite. This may indicate an error in forming the matrix A. There is no routine specifically designed to factorize a symmetric matrix stored in RFP format which is not positive definite; the matrix must be treated as a full symmetric matrix, by calling f07mdf.

7 Accuracy

If uplo='U', the computed factor U is the exact factor of a perturbed matrix A+E, where
|E|c(n)ε|UT||U| ,  
c(n) is a modest linear function of n, and ε is the machine precision.
If uplo='L', a similar statement holds for the computed factor L. It follows that |eij|c(n)εaiiajj.

8 Parallelism and Performance

Background information to multithreading can be found in the Multithreading documentation.
f07wdf is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
f07wdf 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.

9 Further Comments

The total number of floating-point operations is approximately 13n3.
A call to f07wdf may be followed by calls to the routines:
The complex analogue of this routine is f07wrf.

10 Example

This example computes the Cholesky factorization of the matrix A, where
A= ( 4.16 -3.12 0.56 -0.10 -3.12 5.03 -0.83 1.18 0.56 -0.83 0.76 0.34 -0.10 1.18 0.34 1.18 ) ,  
and is stored using RFP format.

10.1 Program Text

Program Text (f07wdfe.f90)

10.2 Program Data

Program Data (f07wdfe.d)

10.3 Program Results

Program Results (f07wdfe.r)