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SUBROUTINE SLANV2( A, B, C, D, RT1R, RT1I, RT2R, RT2I, CS, SN )
*
* -- LAPACK driver routine (version 3.1) --
* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
* November 2006
*
* .. Scalar Arguments ..
REAL A, B, C, CS, D, RT1I, RT1R, RT2I, RT2R, SN
* ..
*
* Purpose
* =======
*
* SLANV2 computes the Schur factorization of a real 2-by-2 nonsymmetric
* matrix in standard form:
*
* [ A B ] = [ CS -SN ] [ AA BB ] [ CS SN ]
* [ C D ] [ SN CS ] [ CC DD ] [-SN CS ]
*
* where either
* 1) CC = 0 so that AA and DD are real eigenvalues of the matrix, or
* 2) AA = DD and BB*CC < 0, so that AA + or - sqrt(BB*CC) are complex
* conjugate eigenvalues.
*
* Arguments
* =========
*
* A (input/output) REAL
* B (input/output) REAL
* C (input/output) REAL
* D (input/output) REAL
* On entry, the elements of the input matrix.
* On exit, they are overwritten by the elements of the
* standardised Schur form.
*
* RT1R (output) REAL
* RT1I (output) REAL
* RT2R (output) REAL
* RT2I (output) REAL
* The real and imaginary parts of the eigenvalues. If the
* eigenvalues are a complex conjugate pair, RT1I > 0.
*
* CS (output) REAL
* SN (output) REAL
* Parameters of the rotation matrix.
*
* Further Details
* ===============
*
* Modified by V. Sima, Research Institute for Informatics, Bucharest,
* Romania, to reduce the risk of cancellation errors,
* when computing real eigenvalues, and to ensure, if possible, that
* abs(RT1R) >= abs(RT2R).
*
* =====================================================================
*
* .. Parameters ..
REAL ZERO, HALF, ONE
PARAMETER ( ZERO = 0.0E+0, HALF = 0.5E+0, ONE = 1.0E+0 )
REAL MULTPL
PARAMETER ( MULTPL = 4.0E+0 )
* ..
* .. Local Scalars ..
REAL AA, BB, BCMAX, BCMIS, CC, CS1, DD, EPS, P, SAB,
$ SAC, SCALE, SIGMA, SN1, TAU, TEMP, Z
* ..
* .. External Functions ..
REAL SLAMCH, SLAPY2
EXTERNAL SLAMCH, SLAPY2
* ..
* .. Intrinsic Functions ..
INTRINSIC ABS, MAX, MIN, SIGN, SQRT
* ..
* .. Executable Statements ..
*
EPS = SLAMCH( 'P' )
IF( C.EQ.ZERO ) THEN
CS = ONE
SN = ZERO
GO TO 10
*
ELSE IF( B.EQ.ZERO ) THEN
*
* Swap rows and columns
*
CS = ZERO
SN = ONE
TEMP = D
D = A
A = TEMP
B = -C
C = ZERO
GO TO 10
ELSE IF( (A-D).EQ.ZERO .AND. SIGN( ONE, B ).NE.
$ SIGN( ONE, C ) ) THEN
CS = ONE
SN = ZERO
GO TO 10
ELSE
*
TEMP = A - D
P = HALF*TEMP
BCMAX = MAX( ABS( B ), ABS( C ) )
BCMIS = MIN( ABS( B ), ABS( C ) )*SIGN( ONE, B )*SIGN( ONE, C )
SCALE = MAX( ABS( P ), BCMAX )
Z = ( P / SCALE )*P + ( BCMAX / SCALE )*BCMIS
*
* If Z is of the order of the machine accuracy, postpone the
* decision on the nature of eigenvalues
*
IF( Z.GE.MULTPL*EPS ) THEN
*
* Real eigenvalues. Compute A and D.
*
Z = P + SIGN( SQRT( SCALE )*SQRT( Z ), P )
A = D + Z
D = D - ( BCMAX / Z )*BCMIS
*
* Compute B and the rotation matrix
*
TAU = SLAPY2( C, Z )
CS = Z / TAU
SN = C / TAU
B = B - C
C = ZERO
ELSE
*
* Complex eigenvalues, or real (almost) equal eigenvalues.
* Make diagonal elements equal.
*
SIGMA = B + C
TAU = SLAPY2( SIGMA, TEMP )
CS = SQRT( HALF*( ONE+ABS( SIGMA ) / TAU ) )
SN = -( P / ( TAU*CS ) )*SIGN( ONE, SIGMA )
*
* Compute [ AA BB ] = [ A B ] [ CS -SN ]
* [ CC DD ] [ C D ] [ SN CS ]
*
AA = A*CS + B*SN
BB = -A*SN + B*CS
CC = C*CS + D*SN
DD = -C*SN + D*CS
*
* Compute [ A B ] = [ CS SN ] [ AA BB ]
* [ C D ] [-SN CS ] [ CC DD ]
*
A = AA*CS + CC*SN
B = BB*CS + DD*SN
C = -AA*SN + CC*CS
D = -BB*SN + DD*CS
*
TEMP = HALF*( A+D )
A = TEMP
D = TEMP
*
IF( C.NE.ZERO ) THEN
IF( B.NE.ZERO ) THEN
IF( SIGN( ONE, B ).EQ.SIGN( ONE, C ) ) THEN
*
* Real eigenvalues: reduce to upper triangular form
*
SAB = SQRT( ABS( B ) )
SAC = SQRT( ABS( C ) )
P = SIGN( SAB*SAC, C )
TAU = ONE / SQRT( ABS( B+C ) )
A = TEMP + P
D = TEMP - P
B = B - C
C = ZERO
CS1 = SAB*TAU
SN1 = SAC*TAU
TEMP = CS*CS1 - SN*SN1
SN = CS*SN1 + SN*CS1
CS = TEMP
END IF
ELSE
B = -C
C = ZERO
TEMP = CS
CS = -SN
SN = TEMP
END IF
END IF
END IF
*
END IF
*
10 CONTINUE
*
* Store eigenvalues in (RT1R,RT1I) and (RT2R,RT2I).
*
RT1R = A
RT2R = D
IF( C.EQ.ZERO ) THEN
RT1I = ZERO
RT2I = ZERO
ELSE
RT1I = SQRT( ABS( B ) )*SQRT( ABS( C ) )
RT2I = -RT1I
END IF
RETURN
*
* End of SLANV2
*
END
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