dlasv2.f (3)  Linux Man Pages
NAME
dlasv2.f 
SYNOPSIS
Functions/Subroutines
subroutine dlasv2 (F, G, H, SSMIN, SSMAX, SNR, CSR, SNL, CSL)
DLASV2 computes the singular value decomposition of a 2by2 triangular matrix.
Function/Subroutine Documentation
subroutine dlasv2 (double precisionF, double precisionG, double precisionH, double precisionSSMIN, double precisionSSMAX, double precisionSNR, double precisionCSR, double precisionSNL, double precisionCSL)
DLASV2 computes the singular value decomposition of a 2by2 triangular matrix.
Purpose:

DLASV2 computes the singular value decomposition of a 2by2 triangular matrix [ F G ] [ 0 H ]. On return, abs(SSMAX) is the larger singular value, abs(SSMIN) is the smaller singular value, and (CSL,SNL) and (CSR,SNR) are the left and right singular vectors for abs(SSMAX), giving the decomposition [ CSL SNL ] [ F G ] [ CSR SNR ] = [ SSMAX 0 ] [SNL CSL ] [ 0 H ] [ SNR CSR ] [ 0 SSMIN ].
Parameters:

F
F is DOUBLE PRECISION The (1,1) element of the 2by2 matrix.
GG is DOUBLE PRECISION The (1,2) element of the 2by2 matrix.
HH is DOUBLE PRECISION The (2,2) element of the 2by2 matrix.
SSMINSSMIN is DOUBLE PRECISION abs(SSMIN) is the smaller singular value.
SSMAXSSMAX is DOUBLE PRECISION abs(SSMAX) is the larger singular value.
SNLSNL is DOUBLE PRECISION
CSLCSL is DOUBLE PRECISION The vector (CSL, SNL) is a unit left singular vector for the singular value abs(SSMAX).
SNRSNR is DOUBLE PRECISION
CSRCSR is DOUBLE PRECISION The vector (CSR, SNR) is a unit right singular vector for the singular value abs(SSMAX).
Author:

Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Date:
 September 2012
Further Details:

Any input parameter may be aliased with any output parameter. Barring over/underflow and assuming a guard digit in subtraction, all output quantities are correct to within a few units in the last place (ulps). In IEEE arithmetic, the code works correctly if one matrix element is infinite. Overflow will not occur unless the largest singular value itself overflows or is within a few ulps of overflow. (On machines with partial overflow, like the Cray, overflow may occur if the largest singular value is within a factor of 2 of overflow.) Underflow is harmless if underflow is gradual. Otherwise, results may correspond to a matrix modified by perturbations of size near the underflow threshold.
Definition at line 139 of file dlasv2.f.
Author
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