Annotation of rpl/lapack/lapack/ztrexc.f, revision 1.1
1.1 ! bertrand 1: SUBROUTINE ZTREXC( COMPQ, N, T, LDT, Q, LDQ, IFST, ILST, INFO )
! 2: *
! 3: * -- LAPACK routine (version 3.2) --
! 4: * -- LAPACK is a software package provided by Univ. of Tennessee, --
! 5: * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
! 6: * November 2006
! 7: *
! 8: * .. Scalar Arguments ..
! 9: CHARACTER COMPQ
! 10: INTEGER IFST, ILST, INFO, LDQ, LDT, N
! 11: * ..
! 12: * .. Array Arguments ..
! 13: COMPLEX*16 Q( LDQ, * ), T( LDT, * )
! 14: * ..
! 15: *
! 16: * Purpose
! 17: * =======
! 18: *
! 19: * ZTREXC reorders the Schur factorization of a complex matrix
! 20: * A = Q*T*Q**H, so that the diagonal element of T with row index IFST
! 21: * is moved to row ILST.
! 22: *
! 23: * The Schur form T is reordered by a unitary similarity transformation
! 24: * Z**H*T*Z, and optionally the matrix Q of Schur vectors is updated by
! 25: * postmultplying it with Z.
! 26: *
! 27: * Arguments
! 28: * =========
! 29: *
! 30: * COMPQ (input) CHARACTER*1
! 31: * = 'V': update the matrix Q of Schur vectors;
! 32: * = 'N': do not update Q.
! 33: *
! 34: * N (input) INTEGER
! 35: * The order of the matrix T. N >= 0.
! 36: *
! 37: * T (input/output) COMPLEX*16 array, dimension (LDT,N)
! 38: * On entry, the upper triangular matrix T.
! 39: * On exit, the reordered upper triangular matrix.
! 40: *
! 41: * LDT (input) INTEGER
! 42: * The leading dimension of the array T. LDT >= max(1,N).
! 43: *
! 44: * Q (input/output) COMPLEX*16 array, dimension (LDQ,N)
! 45: * On entry, if COMPQ = 'V', the matrix Q of Schur vectors.
! 46: * On exit, if COMPQ = 'V', Q has been postmultiplied by the
! 47: * unitary transformation matrix Z which reorders T.
! 48: * If COMPQ = 'N', Q is not referenced.
! 49: *
! 50: * LDQ (input) INTEGER
! 51: * The leading dimension of the array Q. LDQ >= max(1,N).
! 52: *
! 53: * IFST (input) INTEGER
! 54: * ILST (input) INTEGER
! 55: * Specify the reordering of the diagonal elements of T:
! 56: * The element with row index IFST is moved to row ILST by a
! 57: * sequence of transpositions between adjacent elements.
! 58: * 1 <= IFST <= N; 1 <= ILST <= N.
! 59: *
! 60: * INFO (output) INTEGER
! 61: * = 0: successful exit
! 62: * < 0: if INFO = -i, the i-th argument had an illegal value
! 63: *
! 64: * =====================================================================
! 65: *
! 66: * .. Local Scalars ..
! 67: LOGICAL WANTQ
! 68: INTEGER K, M1, M2, M3
! 69: DOUBLE PRECISION CS
! 70: COMPLEX*16 SN, T11, T22, TEMP
! 71: * ..
! 72: * .. External Functions ..
! 73: LOGICAL LSAME
! 74: EXTERNAL LSAME
! 75: * ..
! 76: * .. External Subroutines ..
! 77: EXTERNAL XERBLA, ZLARTG, ZROT
! 78: * ..
! 79: * .. Intrinsic Functions ..
! 80: INTRINSIC DCONJG, MAX
! 81: * ..
! 82: * .. Executable Statements ..
! 83: *
! 84: * Decode and test the input parameters.
! 85: *
! 86: INFO = 0
! 87: WANTQ = LSAME( COMPQ, 'V' )
! 88: IF( .NOT.LSAME( COMPQ, 'N' ) .AND. .NOT.WANTQ ) THEN
! 89: INFO = -1
! 90: ELSE IF( N.LT.0 ) THEN
! 91: INFO = -2
! 92: ELSE IF( LDT.LT.MAX( 1, N ) ) THEN
! 93: INFO = -4
! 94: ELSE IF( LDQ.LT.1 .OR. ( WANTQ .AND. LDQ.LT.MAX( 1, N ) ) ) THEN
! 95: INFO = -6
! 96: ELSE IF( IFST.LT.1 .OR. IFST.GT.N ) THEN
! 97: INFO = -7
! 98: ELSE IF( ILST.LT.1 .OR. ILST.GT.N ) THEN
! 99: INFO = -8
! 100: END IF
! 101: IF( INFO.NE.0 ) THEN
! 102: CALL XERBLA( 'ZTREXC', -INFO )
! 103: RETURN
! 104: END IF
! 105: *
! 106: * Quick return if possible
! 107: *
! 108: IF( N.EQ.1 .OR. IFST.EQ.ILST )
! 109: $ RETURN
! 110: *
! 111: IF( IFST.LT.ILST ) THEN
! 112: *
! 113: * Move the IFST-th diagonal element forward down the diagonal.
! 114: *
! 115: M1 = 0
! 116: M2 = -1
! 117: M3 = 1
! 118: ELSE
! 119: *
! 120: * Move the IFST-th diagonal element backward up the diagonal.
! 121: *
! 122: M1 = -1
! 123: M2 = 0
! 124: M3 = -1
! 125: END IF
! 126: *
! 127: DO 10 K = IFST + M1, ILST + M2, M3
! 128: *
! 129: * Interchange the k-th and (k+1)-th diagonal elements.
! 130: *
! 131: T11 = T( K, K )
! 132: T22 = T( K+1, K+1 )
! 133: *
! 134: * Determine the transformation to perform the interchange.
! 135: *
! 136: CALL ZLARTG( T( K, K+1 ), T22-T11, CS, SN, TEMP )
! 137: *
! 138: * Apply transformation to the matrix T.
! 139: *
! 140: IF( K+2.LE.N )
! 141: $ CALL ZROT( N-K-1, T( K, K+2 ), LDT, T( K+1, K+2 ), LDT, CS,
! 142: $ SN )
! 143: CALL ZROT( K-1, T( 1, K ), 1, T( 1, K+1 ), 1, CS,
! 144: $ DCONJG( SN ) )
! 145: *
! 146: T( K, K ) = T22
! 147: T( K+1, K+1 ) = T11
! 148: *
! 149: IF( WANTQ ) THEN
! 150: *
! 151: * Accumulate transformation in the matrix Q.
! 152: *
! 153: CALL ZROT( N, Q( 1, K ), 1, Q( 1, K+1 ), 1, CS,
! 154: $ DCONJG( SN ) )
! 155: END IF
! 156: *
! 157: 10 CONTINUE
! 158: *
! 159: RETURN
! 160: *
! 161: * End of ZTREXC
! 162: *
! 163: END
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