*> \brief \b ZLAGS2 * * =========== DOCUMENTATION =========== * * Online html documentation available at * http://www.netlib.org/lapack/explore-html/ * *> \htmlonly *> Download ZLAGS2 + dependencies *> *> [TGZ] *> *> [ZIP] *> *> [TXT] *> \endhtmlonly * * Definition: * =========== * * SUBROUTINE ZLAGS2( UPPER, A1, A2, A3, B1, B2, B3, CSU, SNU, CSV, * SNV, CSQ, SNQ ) * * .. Scalar Arguments .. * LOGICAL UPPER * DOUBLE PRECISION A1, A3, B1, B3, CSQ, CSU, CSV * COMPLEX*16 A2, B2, SNQ, SNU, SNV * .. * * *> \par Purpose: * ============= *> *> \verbatim *> *> ZLAGS2 computes 2-by-2 unitary matrices U, V and Q, such *> that if ( UPPER ) then *> *> U**H *A*Q = U**H *( A1 A2 )*Q = ( x 0 ) *> ( 0 A3 ) ( x x ) *> and *> V**H*B*Q = V**H *( B1 B2 )*Q = ( x 0 ) *> ( 0 B3 ) ( x x ) *> *> or if ( .NOT.UPPER ) then *> *> U**H *A*Q = U**H *( A1 0 )*Q = ( x x ) *> ( A2 A3 ) ( 0 x ) *> and *> V**H *B*Q = V**H *( B1 0 )*Q = ( x x ) *> ( B2 B3 ) ( 0 x ) *> where *> *> U = ( CSU SNU ), V = ( CSV SNV ), *> ( -SNU**H CSU ) ( -SNV**H CSV ) *> *> Q = ( CSQ SNQ ) *> ( -SNQ**H CSQ ) *> *> The rows of the transformed A and B are parallel. Moreover, if the *> input 2-by-2 matrix A is not zero, then the transformed (1,1) entry *> of A is not zero. If the input matrices A and B are both not zero, *> then the transformed (2,2) element of B is not zero, except when the *> first rows of input A and B are parallel and the second rows are *> zero. *> \endverbatim * * Arguments: * ========== * *> \param[in] UPPER *> \verbatim *> UPPER is LOGICAL *> = .TRUE.: the input matrices A and B are upper triangular. *> = .FALSE.: the input matrices A and B are lower triangular. *> \endverbatim *> *> \param[in] A1 *> \verbatim *> A1 is DOUBLE PRECISION *> \endverbatim *> *> \param[in] A2 *> \verbatim *> A2 is COMPLEX*16 *> \endverbatim *> *> \param[in] A3 *> \verbatim *> A3 is DOUBLE PRECISION *> On entry, A1, A2 and A3 are elements of the input 2-by-2 *> upper (lower) triangular matrix A. *> \endverbatim *> *> \param[in] B1 *> \verbatim *> B1 is DOUBLE PRECISION *> \endverbatim *> *> \param[in] B2 *> \verbatim *> B2 is COMPLEX*16 *> \endverbatim *> *> \param[in] B3 *> \verbatim *> B3 is DOUBLE PRECISION *> On entry, B1, B2 and B3 are elements of the input 2-by-2 *> upper (lower) triangular matrix B. *> \endverbatim *> *> \param[out] CSU *> \verbatim *> CSU is DOUBLE PRECISION *> \endverbatim *> *> \param[out] SNU *> \verbatim *> SNU is COMPLEX*16 *> The desired unitary matrix U. *> \endverbatim *> *> \param[out] CSV *> \verbatim *> CSV is DOUBLE PRECISION *> \endverbatim *> *> \param[out] SNV *> \verbatim *> SNV is COMPLEX*16 *> The desired unitary matrix V. *> \endverbatim *> *> \param[out] CSQ *> \verbatim *> CSQ is DOUBLE PRECISION *> \endverbatim *> *> \param[out] SNQ *> \verbatim *> SNQ is COMPLEX*16 *> The desired unitary matrix Q. *> \endverbatim * * Authors: * ======== * *> \author Univ. of Tennessee *> \author Univ. of California Berkeley *> \author Univ. of Colorado Denver *> \author NAG Ltd. * *> \ingroup complex16OTHERauxiliary * * ===================================================================== SUBROUTINE ZLAGS2( UPPER, A1, A2, A3, B1, B2, B3, CSU, SNU, CSV, $ SNV, CSQ, SNQ ) * * -- LAPACK auxiliary routine -- * -- LAPACK is a software package provided by Univ. of Tennessee, -- * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..-- * * .. Scalar Arguments .. LOGICAL UPPER DOUBLE PRECISION A1, A3, B1, B3, CSQ, CSU, CSV COMPLEX*16 A2, B2, SNQ, SNU, SNV * .. * * ===================================================================== * * .. Parameters .. DOUBLE PRECISION ZERO, ONE PARAMETER ( ZERO = 0.0D+0, ONE = 1.0D+0 ) * .. * .. Local Scalars .. DOUBLE PRECISION A, AUA11, AUA12, AUA21, AUA22, AVB12, AVB11, $ AVB21, AVB22, CSL, CSR, D, FB, FC, S1, S2, $ SNL, SNR, UA11R, UA22R, VB11R, VB22R COMPLEX*16 B, C, D1, R, T, UA11, UA12, UA21, UA22, VB11, $ VB12, VB21, VB22 * .. * .. External Subroutines .. EXTERNAL DLASV2, ZLARTG * .. * .. Intrinsic Functions .. INTRINSIC ABS, DBLE, DCMPLX, DCONJG, DIMAG * .. * .. Statement Functions .. DOUBLE PRECISION ABS1 * .. * .. Statement Function definitions .. ABS1( T ) = ABS( DBLE( T ) ) + ABS( DIMAG( T ) ) * .. * .. Executable Statements .. * IF( UPPER ) THEN * * Input matrices A and B are upper triangular matrices * * Form matrix C = A*adj(B) = ( a b ) * ( 0 d ) * A = A1*B3 D = A3*B1 B = A2*B1 - A1*B2 FB = ABS( B ) * * Transform complex 2-by-2 matrix C to real matrix by unitary * diagonal matrix diag(1,D1). * D1 = ONE IF( FB.NE.ZERO ) $ D1 = B / FB * * The SVD of real 2 by 2 triangular C * * ( CSL -SNL )*( A B )*( CSR SNR ) = ( R 0 ) * ( SNL CSL ) ( 0 D ) ( -SNR CSR ) ( 0 T ) * CALL DLASV2( A, FB, D, S1, S2, SNR, CSR, SNL, CSL ) * IF( ABS( CSL ).GE.ABS( SNL ) .OR. ABS( CSR ).GE.ABS( SNR ) ) $ THEN * * Compute the (1,1) and (1,2) elements of U**H *A and V**H *B, * and (1,2) element of |U|**H *|A| and |V|**H *|B|. * UA11R = CSL*A1 UA12 = CSL*A2 + D1*SNL*A3 * VB11R = CSR*B1 VB12 = CSR*B2 + D1*SNR*B3 * AUA12 = ABS( CSL )*ABS1( A2 ) + ABS( SNL )*ABS( A3 ) AVB12 = ABS( CSR )*ABS1( B2 ) + ABS( SNR )*ABS( B3 ) * * zero (1,2) elements of U**H *A and V**H *B * IF( ( ABS( UA11R )+ABS1( UA12 ) ).EQ.ZERO ) THEN CALL ZLARTG( -DCMPLX( VB11R ), DCONJG( VB12 ), CSQ, SNQ, $ R ) ELSE IF( ( ABS( VB11R )+ABS1( VB12 ) ).EQ.ZERO ) THEN CALL ZLARTG( -DCMPLX( UA11R ), DCONJG( UA12 ), CSQ, SNQ, $ R ) ELSE IF( AUA12 / ( ABS( UA11R )+ABS1( UA12 ) ).LE.AVB12 / $ ( ABS( VB11R )+ABS1( VB12 ) ) ) THEN CALL ZLARTG( -DCMPLX( UA11R ), DCONJG( UA12 ), CSQ, SNQ, $ R ) ELSE CALL ZLARTG( -DCMPLX( VB11R ), DCONJG( VB12 ), CSQ, SNQ, $ R ) END IF * CSU = CSL SNU = -D1*SNL CSV = CSR SNV = -D1*SNR * ELSE * * Compute the (2,1) and (2,2) elements of U**H *A and V**H *B, * and (2,2) element of |U|**H *|A| and |V|**H *|B|. * UA21 = -DCONJG( D1 )*SNL*A1 UA22 = -DCONJG( D1 )*SNL*A2 + CSL*A3 * VB21 = -DCONJG( D1 )*SNR*B1 VB22 = -DCONJG( D1 )*SNR*B2 + CSR*B3 * AUA22 = ABS( SNL )*ABS1( A2 ) + ABS( CSL )*ABS( A3 ) AVB22 = ABS( SNR )*ABS1( B2 ) + ABS( CSR )*ABS( B3 ) * * zero (2,2) elements of U**H *A and V**H *B, and then swap. * IF( ( ABS1( UA21 )+ABS1( UA22 ) ).EQ.ZERO ) THEN CALL ZLARTG( -DCONJG( VB21 ), DCONJG( VB22 ), CSQ, SNQ, $ R ) ELSE IF( ( ABS1( VB21 )+ABS( VB22 ) ).EQ.ZERO ) THEN CALL ZLARTG( -DCONJG( UA21 ), DCONJG( UA22 ), CSQ, SNQ, $ R ) ELSE IF( AUA22 / ( ABS1( UA21 )+ABS1( UA22 ) ).LE.AVB22 / $ ( ABS1( VB21 )+ABS1( VB22 ) ) ) THEN CALL ZLARTG( -DCONJG( UA21 ), DCONJG( UA22 ), CSQ, SNQ, $ R ) ELSE CALL ZLARTG( -DCONJG( VB21 ), DCONJG( VB22 ), CSQ, SNQ, $ R ) END IF * CSU = SNL SNU = D1*CSL CSV = SNR SNV = D1*CSR * END IF * ELSE * * Input matrices A and B are lower triangular matrices * * Form matrix C = A*adj(B) = ( a 0 ) * ( c d ) * A = A1*B3 D = A3*B1 C = A2*B3 - A3*B2 FC = ABS( C ) * * Transform complex 2-by-2 matrix C to real matrix by unitary * diagonal matrix diag(d1,1). * D1 = ONE IF( FC.NE.ZERO ) $ D1 = C / FC * * The SVD of real 2 by 2 triangular C * * ( CSL -SNL )*( A 0 )*( CSR SNR ) = ( R 0 ) * ( SNL CSL ) ( C D ) ( -SNR CSR ) ( 0 T ) * CALL DLASV2( A, FC, D, S1, S2, SNR, CSR, SNL, CSL ) * IF( ABS( CSR ).GE.ABS( SNR ) .OR. ABS( CSL ).GE.ABS( SNL ) ) $ THEN * * Compute the (2,1) and (2,2) elements of U**H *A and V**H *B, * and (2,1) element of |U|**H *|A| and |V|**H *|B|. * UA21 = -D1*SNR*A1 + CSR*A2 UA22R = CSR*A3 * VB21 = -D1*SNL*B1 + CSL*B2 VB22R = CSL*B3 * AUA21 = ABS( SNR )*ABS( A1 ) + ABS( CSR )*ABS1( A2 ) AVB21 = ABS( SNL )*ABS( B1 ) + ABS( CSL )*ABS1( B2 ) * * zero (2,1) elements of U**H *A and V**H *B. * IF( ( ABS1( UA21 )+ABS( UA22R ) ).EQ.ZERO ) THEN CALL ZLARTG( DCMPLX( VB22R ), VB21, CSQ, SNQ, R ) ELSE IF( ( ABS1( VB21 )+ABS( VB22R ) ).EQ.ZERO ) THEN CALL ZLARTG( DCMPLX( UA22R ), UA21, CSQ, SNQ, R ) ELSE IF( AUA21 / ( ABS1( UA21 )+ABS( UA22R ) ).LE.AVB21 / $ ( ABS1( VB21 )+ABS( VB22R ) ) ) THEN CALL ZLARTG( DCMPLX( UA22R ), UA21, CSQ, SNQ, R ) ELSE CALL ZLARTG( DCMPLX( VB22R ), VB21, CSQ, SNQ, R ) END IF * CSU = CSR SNU = -DCONJG( D1 )*SNR CSV = CSL SNV = -DCONJG( D1 )*SNL * ELSE * * Compute the (1,1) and (1,2) elements of U**H *A and V**H *B, * and (1,1) element of |U|**H *|A| and |V|**H *|B|. * UA11 = CSR*A1 + DCONJG( D1 )*SNR*A2 UA12 = DCONJG( D1 )*SNR*A3 * VB11 = CSL*B1 + DCONJG( D1 )*SNL*B2 VB12 = DCONJG( D1 )*SNL*B3 * AUA11 = ABS( CSR )*ABS( A1 ) + ABS( SNR )*ABS1( A2 ) AVB11 = ABS( CSL )*ABS( B1 ) + ABS( SNL )*ABS1( B2 ) * * zero (1,1) elements of U**H *A and V**H *B, and then swap. * IF( ( ABS1( UA11 )+ABS1( UA12 ) ).EQ.ZERO ) THEN CALL ZLARTG( VB12, VB11, CSQ, SNQ, R ) ELSE IF( ( ABS1( VB11 )+ABS1( VB12 ) ).EQ.ZERO ) THEN CALL ZLARTG( UA12, UA11, CSQ, SNQ, R ) ELSE IF( AUA11 / ( ABS1( UA11 )+ABS1( UA12 ) ).LE.AVB11 / $ ( ABS1( VB11 )+ABS1( VB12 ) ) ) THEN CALL ZLARTG( UA12, UA11, CSQ, SNQ, R ) ELSE CALL ZLARTG( VB12, VB11, CSQ, SNQ, R ) END IF * CSU = SNR SNU = DCONJG( D1 )*CSR CSV = SNL SNV = DCONJG( D1 )*CSL * END IF * END IF * RETURN * * End of ZLAGS2 * END