LAPACK
3.4.2
LAPACK: Linear Algebra PACKage

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Functions/Subroutines  
subroutine  dlag2 (A, LDA, B, LDB, SAFMIN, SCALE1, SCALE2, WR1, WR2, WI) 
DLAG2 computes the eigenvalues of a 2by2 generalized eigenvalue problem, with scaling as necessary to avoid over/underflow. 
subroutine dlag2  (  double precision, dimension( lda, * )  A, 
integer  LDA,  
double precision, dimension( ldb, * )  B,  
integer  LDB,  
double precision  SAFMIN,  
double precision  SCALE1,  
double precision  SCALE2,  
double precision  WR1,  
double precision  WR2,  
double precision  WI  
) 
DLAG2 computes the eigenvalues of a 2by2 generalized eigenvalue problem, with scaling as necessary to avoid over/underflow.
Download DLAG2 + dependencies [TGZ] [ZIP] [TXT]DLAG2 computes the eigenvalues of a 2 x 2 generalized eigenvalue problem A  w B, with scaling as necessary to avoid over/underflow. The scaling factor "s" results in a modified eigenvalue equation s A  w B where s is a nonnegative scaling factor chosen so that w, w B, and s A do not overflow and, if possible, do not underflow, either.
[in]  A  A is DOUBLE PRECISION array, dimension (LDA, 2) On entry, the 2 x 2 matrix A. It is assumed that its 1norm is less than 1/SAFMIN. Entries less than sqrt(SAFMIN)*norm(A) are subject to being treated as zero. 
[in]  LDA  LDA is INTEGER The leading dimension of the array A. LDA >= 2. 
[in]  B  B is DOUBLE PRECISION array, dimension (LDB, 2) On entry, the 2 x 2 upper triangular matrix B. It is assumed that the onenorm of B is less than 1/SAFMIN. The diagonals should be at least sqrt(SAFMIN) times the largest element of B (in absolute value); if a diagonal is smaller than that, then +/ sqrt(SAFMIN) will be used instead of that diagonal. 
[in]  LDB  LDB is INTEGER The leading dimension of the array B. LDB >= 2. 
[in]  SAFMIN  SAFMIN is DOUBLE PRECISION The smallest positive number s.t. 1/SAFMIN does not overflow. (This should always be DLAMCH('S')  it is an argument in order to avoid having to call DLAMCH frequently.) 
[out]  SCALE1  SCALE1 is DOUBLE PRECISION A scaling factor used to avoid over/underflow in the eigenvalue equation which defines the first eigenvalue. If the eigenvalues are complex, then the eigenvalues are ( WR1 +/ WI i ) / SCALE1 (which may lie outside the exponent range of the machine), SCALE1=SCALE2, and SCALE1 will always be positive. If the eigenvalues are real, then the first (real) eigenvalue is WR1 / SCALE1 , but this may overflow or underflow, and in fact, SCALE1 may be zero or less than the underflow threshhold if the exact eigenvalue is sufficiently large. 
[out]  SCALE2  SCALE2 is DOUBLE PRECISION A scaling factor used to avoid over/underflow in the eigenvalue equation which defines the second eigenvalue. If the eigenvalues are complex, then SCALE2=SCALE1. If the eigenvalues are real, then the second (real) eigenvalue is WR2 / SCALE2 , but this may overflow or underflow, and in fact, SCALE2 may be zero or less than the underflow threshhold if the exact eigenvalue is sufficiently large. 
[out]  WR1  WR1 is DOUBLE PRECISION If the eigenvalue is real, then WR1 is SCALE1 times the eigenvalue closest to the (2,2) element of A B**(1). If the eigenvalue is complex, then WR1=WR2 is SCALE1 times the real part of the eigenvalues. 
[out]  WR2  WR2 is DOUBLE PRECISION If the eigenvalue is real, then WR2 is SCALE2 times the other eigenvalue. If the eigenvalue is complex, then WR1=WR2 is SCALE1 times the real part of the eigenvalues. 
[out]  WI  WI is DOUBLE PRECISION If the eigenvalue is real, then WI is zero. If the eigenvalue is complex, then WI is SCALE1 times the imaginary part of the eigenvalues. WI will always be nonnegative. 
Definition at line 156 of file dlag2.f.