LAPACK
3.4.2
LAPACK: Linear Algebra PACKage

Go to the source code of this file.
Functions/Subroutines  
subroutine  dlaln2 (LTRANS, NA, NW, SMIN, CA, A, LDA, D1, D2, B, LDB, WR, WI, X, LDX, SCALE, XNORM, INFO) 
DLALN2 solves a 1by1 or 2by2 linear system of equations of the specified form. 
subroutine dlaln2  (  logical  LTRANS, 
integer  NA,  
integer  NW,  
double precision  SMIN,  
double precision  CA,  
double precision, dimension( lda, * )  A,  
integer  LDA,  
double precision  D1,  
double precision  D2,  
double precision, dimension( ldb, * )  B,  
integer  LDB,  
double precision  WR,  
double precision  WI,  
double precision, dimension( ldx, * )  X,  
integer  LDX,  
double precision  SCALE,  
double precision  XNORM,  
integer  INFO  
) 
DLALN2 solves a 1by1 or 2by2 linear system of equations of the specified form.
Download DLALN2 + dependencies [TGZ] [ZIP] [TXT]DLALN2 solves a system of the form (ca A  w D ) X = s B or (ca A**T  w D) X = s B with possible scaling ("s") and perturbation of A. (A**T means Atranspose.) A is an NA x NA real matrix, ca is a real scalar, D is an NA x NA real diagonal matrix, w is a real or complex value, and X and B are NA x 1 matrices  real if w is real, complex if w is complex. NA may be 1 or 2. If w is complex, X and B are represented as NA x 2 matrices, the first column of each being the real part and the second being the imaginary part. "s" is a scaling factor (.LE. 1), computed by DLALN2, which is so chosen that X can be computed without overflow. X is further scaled if necessary to assure that norm(ca A  w D)*norm(X) is less than overflow. If both singular values of (ca A  w D) are less than SMIN, SMIN*identity will be used instead of (ca A  w D). If only one singular value is less than SMIN, one element of (ca A  w D) will be perturbed enough to make the smallest singular value roughly SMIN. If both singular values are at least SMIN, (ca A  w D) will not be perturbed. In any case, the perturbation will be at most some small multiple of max( SMIN, ulp*norm(ca A  w D) ). The singular values are computed by infinitynorm approximations, and thus will only be correct to a factor of 2 or so. Note: all input quantities are assumed to be smaller than overflow by a reasonable factor. (See BIGNUM.)
[in]  LTRANS  LTRANS is LOGICAL =.TRUE.: Atranspose will be used. =.FALSE.: A will be used (not transposed.) 
[in]  NA  NA is INTEGER The size of the matrix A. It may (only) be 1 or 2. 
[in]  NW  NW is INTEGER 1 if "w" is real, 2 if "w" is complex. It may only be 1 or 2. 
[in]  SMIN  SMIN is DOUBLE PRECISION The desired lower bound on the singular values of A. This should be a safe distance away from underflow or overflow, say, between (underflow/machine precision) and (machine precision * overflow ). (See BIGNUM and ULP.) 
[in]  CA  CA is DOUBLE PRECISION The coefficient c, which A is multiplied by. 
[in]  A  A is DOUBLE PRECISION array, dimension (LDA,NA) The NA x NA matrix A. 
[in]  LDA  LDA is INTEGER The leading dimension of A. It must be at least NA. 
[in]  D1  D1 is DOUBLE PRECISION The 1,1 element in the diagonal matrix D. 
[in]  D2  D2 is DOUBLE PRECISION The 2,2 element in the diagonal matrix D. Not used if NW=1. 
[in]  B  B is DOUBLE PRECISION array, dimension (LDB,NW) The NA x NW matrix B (righthand side). If NW=2 ("w" is complex), column 1 contains the real part of B and column 2 contains the imaginary part. 
[in]  LDB  LDB is INTEGER The leading dimension of B. It must be at least NA. 
[in]  WR  WR is DOUBLE PRECISION The real part of the scalar "w". 
[in]  WI  WI is DOUBLE PRECISION The imaginary part of the scalar "w". Not used if NW=1. 
[out]  X  X is DOUBLE PRECISION array, dimension (LDX,NW) The NA x NW matrix X (unknowns), as computed by DLALN2. If NW=2 ("w" is complex), on exit, column 1 will contain the real part of X and column 2 will contain the imaginary part. 
[in]  LDX  LDX is INTEGER The leading dimension of X. It must be at least NA. 
[out]  SCALE  SCALE is DOUBLE PRECISION The scale factor that B must be multiplied by to insure that overflow does not occur when computing X. Thus, (ca A  w D) X will be SCALE*B, not B (ignoring perturbations of A.) It will be at most 1. 
[out]  XNORM  XNORM is DOUBLE PRECISION The infinitynorm of X, when X is regarded as an NA x NW real matrix. 
[out]  INFO  INFO is INTEGER An error flag. It will be set to zero if no error occurs, a negative number if an argument is in error, or a positive number if ca A  w D had to be perturbed. The possible values are: = 0: No error occurred, and (ca A  w D) did not have to be perturbed. = 1: (ca A  w D) had to be perturbed to make its smallest (or only) singular value greater than SMIN. NOTE: In the interests of speed, this routine does not check the inputs for errors. 
Definition at line 218 of file dlaln2.f.