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SUBROUTINE SLAGS2( UPPER, A1, A2, A3, B1, B2, B3, CSU, SNU, CSV,
$ SNV, CSQ, SNQ )
*
* -- LAPACK auxiliary routine (version 3.2) --
* -- LAPACK is a software package provided by Univ. of Tennessee, --
* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
* November 2006
*
* .. Scalar Arguments ..
LOGICAL UPPER
REAL A1, A2, A3, B1, B2, B3, CSQ, CSU, CSV, SNQ,
$ SNU, SNV
* ..
*
* Purpose
* =======
*
* SLAGS2 computes 2-by-2 orthogonal matrices U, V and Q, such
* that if ( UPPER ) then
*
* U**T *A*Q = U**T *( A1 A2 )*Q = ( x 0 )
* ( 0 A3 ) ( x x )
* and
* V**T*B*Q = V**T *( B1 B2 )*Q = ( x 0 )
* ( 0 B3 ) ( x x )
*
* or if ( .NOT.UPPER ) then
*
* U**T *A*Q = U**T *( A1 0 )*Q = ( x x )
* ( A2 A3 ) ( 0 x )
* and
* V**T*B*Q = V**T*( B1 0 )*Q = ( x x )
* ( B2 B3 ) ( 0 x )
*
* The rows of the transformed A and B are parallel, where
*
* U = ( CSU SNU ), V = ( CSV SNV ), Q = ( CSQ SNQ )
* ( -SNU CSU ) ( -SNV CSV ) ( -SNQ CSQ )
*
* Z**T denotes the transpose of Z.
*
*
* Arguments
* =========
*
* UPPER (input) LOGICAL
* = .TRUE.: the input matrices A and B are upper triangular.
* = .FALSE.: the input matrices A and B are lower triangular.
*
* A1 (input) REAL
* A2 (input) REAL
* A3 (input) REAL
* On entry, A1, A2 and A3 are elements of the input 2-by-2
* upper (lower) triangular matrix A.
*
* B1 (input) REAL
* B2 (input) REAL
* B3 (input) REAL
* On entry, B1, B2 and B3 are elements of the input 2-by-2
* upper (lower) triangular matrix B.
*
* CSU (output) REAL
* SNU (output) REAL
* The desired orthogonal matrix U.
*
* CSV (output) REAL
* SNV (output) REAL
* The desired orthogonal matrix V.
*
* CSQ (output) REAL
* SNQ (output) REAL
* The desired orthogonal matrix Q.
*
* =====================================================================
*
* .. Parameters ..
REAL ZERO
PARAMETER ( ZERO = 0.0E+0 )
* ..
* .. Local Scalars ..
REAL A, AUA11, AUA12, AUA21, AUA22, AVB11, AVB12,
$ AVB21, AVB22, CSL, CSR, D, S1, S2, SNL,
$ SNR, UA11R, UA22R, VB11R, VB22R, B, C, R, UA11,
$ UA12, UA21, UA22, VB11, VB12, VB21, VB22
* ..
* .. External Subroutines ..
EXTERNAL SLARTG, SLASV2
* ..
* .. Intrinsic Functions ..
INTRINSIC ABS
* ..
* .. 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
*
* 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 SLASV2( A, B, 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**T *A and V**T *B,
* and (1,2) element of |U|**T *|A| and |V|**T *|B|.
*
UA11R = CSL*A1
UA12 = CSL*A2 + SNL*A3
*
VB11R = CSR*B1
VB12 = CSR*B2 + SNR*B3
*
AUA12 = ABS( CSL )*ABS( A2 ) + ABS( SNL )*ABS( A3 )
AVB12 = ABS( CSR )*ABS( B2 ) + ABS( SNR )*ABS( B3 )
*
* zero (1,2) elements of U**T *A and V**T *B
*
IF( ( ABS( UA11R )+ABS( UA12 ) ).NE.ZERO ) THEN
IF( AUA12 / ( ABS( UA11R )+ABS( UA12 ) ).LE.AVB12 /
$ ( ABS( VB11R )+ABS( VB12 ) ) ) THEN
CALL SLARTG( -UA11R, UA12, CSQ, SNQ, R )
ELSE
CALL SLARTG( -VB11R, VB12, CSQ, SNQ, R )
END IF
ELSE
CALL SLARTG( -VB11R, VB12, CSQ, SNQ, R )
END IF
*
CSU = CSL
SNU = -SNL
CSV = CSR
SNV = -SNR
*
ELSE
*
* Compute the (2,1) and (2,2) elements of U**T *A and V**T *B,
* and (2,2) element of |U|**T *|A| and |V|**T *|B|.
*
UA21 = -SNL*A1
UA22 = -SNL*A2 + CSL*A3
*
VB21 = -SNR*B1
VB22 = -SNR*B2 + CSR*B3
*
AUA22 = ABS( SNL )*ABS( A2 ) + ABS( CSL )*ABS( A3 )
AVB22 = ABS( SNR )*ABS( B2 ) + ABS( CSR )*ABS( B3 )
*
* zero (2,2) elements of U**T*A and V**T*B, and then swap.
*
IF( ( ABS( UA21 )+ABS( UA22 ) ).NE.ZERO ) THEN
IF( AUA22 / ( ABS( UA21 )+ABS( UA22 ) ).LE.AVB22 /
$ ( ABS( VB21 )+ABS( VB22 ) ) ) THEN
CALL SLARTG( -UA21, UA22, CSQ, SNQ, R )
ELSE
CALL SLARTG( -VB21, VB22, CSQ, SNQ, R )
END IF
ELSE
CALL SLARTG( -VB21, VB22, CSQ, SNQ, R )
END IF
*
CSU = SNL
SNU = CSL
CSV = SNR
SNV = 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
*
* 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 SLASV2( A, C, 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**T *A and V**T *B,
* and (2,1) element of |U|**T *|A| and |V|**T *|B|.
*
UA21 = -SNR*A1 + CSR*A2
UA22R = CSR*A3
*
VB21 = -SNL*B1 + CSL*B2
VB22R = CSL*B3
*
AUA21 = ABS( SNR )*ABS( A1 ) + ABS( CSR )*ABS( A2 )
AVB21 = ABS( SNL )*ABS( B1 ) + ABS( CSL )*ABS( B2 )
*
* zero (2,1) elements of U**T *A and V**T *B.
*
IF( ( ABS( UA21 )+ABS( UA22R ) ).NE.ZERO ) THEN
IF( AUA21 / ( ABS( UA21 )+ABS( UA22R ) ).LE.AVB21 /
$ ( ABS( VB21 )+ABS( VB22R ) ) ) THEN
CALL SLARTG( UA22R, UA21, CSQ, SNQ, R )
ELSE
CALL SLARTG( VB22R, VB21, CSQ, SNQ, R )
END IF
ELSE
CALL SLARTG( VB22R, VB21, CSQ, SNQ, R )
END IF
*
CSU = CSR
SNU = -SNR
CSV = CSL
SNV = -SNL
*
ELSE
*
* Compute the (1,1) and (1,2) elements of U**T *A and V**T *B,
* and (1,1) element of |U|**T *|A| and |V|**T *|B|.
*
UA11 = CSR*A1 + SNR*A2
UA12 = SNR*A3
*
VB11 = CSL*B1 + SNL*B2
VB12 = SNL*B3
*
AUA11 = ABS( CSR )*ABS( A1 ) + ABS( SNR )*ABS( A2 )
AVB11 = ABS( CSL )*ABS( B1 ) + ABS( SNL )*ABS( B2 )
*
* zero (1,1) elements of U**T*A and V**T*B, and then swap.
*
IF( ( ABS( UA11 )+ABS( UA12 ) ).NE.ZERO ) THEN
IF( AUA11 / ( ABS( UA11 )+ABS( UA12 ) ).LE.AVB11 /
$ ( ABS( VB11 )+ABS( VB12 ) ) ) THEN
CALL SLARTG( UA12, UA11, CSQ, SNQ, R )
ELSE
CALL SLARTG( VB12, VB11, CSQ, SNQ, R )
END IF
ELSE
CALL SLARTG( VB12, VB11, CSQ, SNQ, R )
END IF
*
CSU = SNR
SNU = CSR
CSV = SNL
SNV = CSL
*
END IF
*
END IF
*
RETURN
*
* End of SLAGS2
*
END
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