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NAME SLALN2 - solve a system of the form (ca A - w D ) X = s B or (ca A' - w D) X = s B with possible scaling ("s") and per- turbation of A SYNOPSIS SUBROUTINE SLALN2( LTRANS, NA, NW, SMIN, CA, A, LDA, D1, D2, B, LDB, WR, WI, X, LDX, SCALE, XNORM, INFO ) LOGICAL LTRANS INTEGER INFO, LDA, LDB, LDX, NA, NW REAL CA, D1, D2, SCALE, SMIN, WI, WR, XNORM REAL A( LDA, * ), B( LDB, * ), X( LDX, * ) PURPOSE SLALN2 solves a system of the form (ca A - w D ) X = s B or (ca A' - w D) X = s B with possible scaling ("s") and per- turbation of A. (A' means A-transpose.) 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 SLALN2, 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 com- puted by infinity-norm 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.) ARGUMENTS LTRANS (input) LOGICAL =.TRUE.: A-transpose will be used. =.FALSE.: A will be used (not transposed.) NA (input) INTEGER The size of the matrix A. It may (only) be 1 or 2. NW (input) INTEGER 1 if "w" is real, 2 if "w" is complex. It may only be 1 or 2. SMIN (input) REAL 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 preci- sion) and (machine precision * overflow ). (See BIGNUM and ULP.) CA (input) REAL The coefficient c, which A is multiplied by. A (input) REAL array, dimension (LDA,NA) The NA x NA matrix A. LDA (input) INTEGER The leading dimension of A. It must be at least NA. D1 (input) REAL The 1,1 entry in the diagonal matrix D. D2 (input) REAL The 2,2 entry in the diagonal matrix D. Not used if NW=1. B (input) REAL array, dimension (LDB,NW) The NA x NW matrix B (right-hand side). If NW=2 ("w" is complex), column 1 contains the real part of B and column 2 contains the imaginary part. LDB (input) INTEGER The leading dimension of B. It must be at least NA. WR (input) REAL The real part of the scalar "w". WI (input) REAL The imaginary part of the scalar "w". Not used if NW=1. X (output) REAL array, dimension (LDX,NW) The NA x NW matrix X (unknowns), as computed by SLALN2. 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. LDX (input) INTEGER The leading dimension of X. It must be at least NA. SCALE (output) REAL 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. XNORM (output) REAL The infinity-norm of X, when X is regarded as an NA x NW real matrix. INFO (output) 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.