dheqr.f
SUBROUTINE DHEQR (A, LDA, N, Q, INFO, IJOB)
C***BEGIN PROLOGUE DHEQR
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Warning: this routine is not intended to be user-callable.
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C***SUBSIDIARY
C***PURPOSE Internal routine for DGMRES.
C***LIBRARY SLATEC (SLAP)
C***CATEGORY D2A4, D2B4
C***TYPE DOUBLE PRECISION (SHEQR-S, DHEQR-D)
C***KEYWORDS GENERALIZED MINIMUM RESIDUAL, ITERATIVE PRECONDITION,
C NON-SYMMETRIC LINEAR SYSTEM, SLAP, SPARSE
C***AUTHOR Brown, Peter, (LLNL), pnbrown@llnl.gov
C Hindmarsh, Alan, (LLNL), alanh@llnl.gov
C Seager, Mark K., (LLNL), seager@llnl.gov
C Lawrence Livermore National Laboratory
C PO Box 808, L-60
C Livermore, CA 94550 (510) 423-3141
C***DESCRIPTION
C This routine performs a QR decomposition of an upper
C Hessenberg matrix A using Givens rotations. There are two
C options available: 1) Performing a fresh decomposition 2)
C updating the QR factors by adding a row and a column to the
C matrix A.
C
C *Usage:
C INTEGER LDA, N, INFO, IJOB
C DOUBLE PRECISION A(LDA,N), Q(2*N)
C
C CALL DHEQR(A, LDA, N, Q, INFO, IJOB)
C
C *Arguments:
C A :INOUT Double Precision A(LDA,N)
C On input, the matrix to be decomposed.
C On output, the upper triangular matrix R.
C The factorization can be written Q*A = R, where
C Q is a product of Givens rotations and R is upper
C triangular.
C LDA :IN Integer
C The leading dimension of the array A.
C N :IN Integer
C A is an (N+1) by N Hessenberg matrix.
C Q :OUT Double Precision Q(2*N)
C The factors c and s of each Givens rotation used
C in decomposing A.
C INFO :OUT Integer
C = 0 normal value.
C = K if A(K,K) .eq. 0.0 . This is not an error
C condition for this subroutine, but it does
C indicate that DHELS will divide by zero
C if called.
C IJOB :IN Integer
C = 1 means that a fresh decomposition of the
C matrix A is desired.
C .ge. 2 means that the current decomposition of A
C will be updated by the addition of a row
C and a column.
C
C***SEE ALSO DGMRES
C***ROUTINES CALLED (NONE)
C***REVISION HISTORY (YYMMDD)
C 890404 DATE WRITTEN
C 890404 Previous REVISION DATE
C 890915 Made changes requested at July 1989 CML Meeting. (MKS)
C 890922 Numerous changes to prologue to make closer to SLATEC
C standard. (FNF)
C 890929 Numerous changes to reduce SP/DP differences. (FNF)
C 910411 Prologue converted to Version 4.0 format. (BAB)
C 910506 Made subsidiary to DGMRES. (FNF)
C 920511 Added complete declaration section. (WRB)
C***END PROLOGUE DHEQR