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SUBROUTINE SGTT01( N, DL, D, DU, DLF, DF, DUF, DU2, IPIV, WORK,
$ LDWORK, RWORK, RESID )
*
* -- LAPACK test routine (version 3.0) --
* Univ. of Tennessee, Univ. of California Berkeley, NAG Ltd.,
* Courant Institute, Argonne National Lab, and Rice University
* February 29, 1992
*
* .. Scalar Arguments ..
INTEGER LDWORK, N
REAL RESID
* ..
* .. Array Arguments ..
INTEGER IPIV( * )
REAL D( * ), DF( * ), DL( * ), DLF( * ), DU( * ),
$ DU2( * ), DUF( * ), RWORK( * ),
$ WORK( LDWORK, * )
* ..
*
* Purpose
* =======
*
* SGTT01 reconstructs a tridiagonal matrix A from its LU factorization
* and computes the residual
* norm(L*U - A) / ( norm(A) * EPS ),
* where EPS is the machine epsilon.
*
* Arguments
* =========
*
* N (input) INTEGTER
* The order of the matrix A. N >= 0.
*
* DL (input) REAL array, dimension (N-1)
* The (n-1) sub-diagonal elements of A.
*
* D (input) REAL array, dimension (N)
* The diagonal elements of A.
*
* DU (input) REAL array, dimension (N-1)
* The (n-1) super-diagonal elements of A.
*
* DLF (input) REAL array, dimension (N-1)
* The (n-1) multipliers that define the matrix L from the
* LU factorization of A.
*
* DF (input) REAL array, dimension (N)
* The n diagonal elements of the upper triangular matrix U from
* the LU factorization of A.
*
* DUF (input) REAL array, dimension (N-1)
* The (n-1) elements of the first super-diagonal of U.
*
* DU2F (input) REAL array, dimension (N-2)
* The (n-2) elements of the second super-diagonal of U.
*
* IPIV (input) INTEGER array, dimension (N)
* The pivot indices; for 1 <= i <= n, row i of the matrix was
* interchanged with row IPIV(i). IPIV(i) will always be either
* i or i+1; IPIV(i) = i indicates a row interchange was not
* required.
*
* WORK (workspace) REAL array, dimension (LDWORK,N)
*
* LDWORK (input) INTEGER
* The leading dimension of the array WORK. LDWORK >= max(1,N).
*
* RWORK (workspace) REAL array, dimension (N)
*
* RESID (output) REAL
* The scaled residual: norm(L*U - A) / (norm(A) * EPS)
*
* =====================================================================
*
* .. Parameters ..
REAL ONE, ZERO
PARAMETER ( ONE = 1.0E+0, ZERO = 0.0E+0 )
* ..
* .. Local Scalars ..
INTEGER I, IP, J, LASTJ
REAL ANORM, EPS, LI
* ..
* .. External Functions ..
REAL SLAMCH, SLANGT, SLANHS
EXTERNAL SLAMCH, SLANGT, SLANHS
* ..
* .. Intrinsic Functions ..
INTRINSIC MIN
* ..
* .. External Subroutines ..
EXTERNAL SAXPY, SSWAP
* ..
* .. Executable Statements ..
*
* Quick return if possible
*
IF( N.LE.0 ) THEN
RESID = ZERO
RETURN
END IF
*
EPS = SLAMCH( 'Epsilon' )
*
* Copy the matrix U to WORK.
*
DO 20 J = 1, N
DO 10 I = 1, N
WORK( I, J ) = ZERO
10 CONTINUE
20 CONTINUE
DO 30 I = 1, N
IF( I.EQ.1 ) THEN
WORK( I, I ) = DF( I )
IF( N.GE.2 )
$ WORK( I, I+1 ) = DUF( I )
IF( N.GE.3 )
$ WORK( I, I+2 ) = DU2( I )
ELSE IF( I.EQ.N ) THEN
WORK( I, I ) = DF( I )
ELSE
WORK( I, I ) = DF( I )
WORK( I, I+1 ) = DUF( I )
IF( I.LT.N-1 )
$ WORK( I, I+2 ) = DU2( I )
END IF
30 CONTINUE
*
* Multiply on the left by L.
*
LASTJ = N
DO 40 I = N - 1, 1, -1
LI = DLF( I )
CALL SAXPY( LASTJ-I+1, LI, WORK( I, I ), LDWORK,
$ WORK( I+1, I ), LDWORK )
IP = IPIV( I )
IF( IP.EQ.I ) THEN
LASTJ = MIN( I+2, N )
ELSE
CALL SSWAP( LASTJ-I+1, WORK( I, I ), LDWORK, WORK( I+1, I ),
$ LDWORK )
END IF
40 CONTINUE
*
* Subtract the matrix A.
*
WORK( 1, 1 ) = WORK( 1, 1 ) - D( 1 )
IF( N.GT.1 ) THEN
WORK( 1, 2 ) = WORK( 1, 2 ) - DU( 1 )
WORK( N, N-1 ) = WORK( N, N-1 ) - DL( N-1 )
WORK( N, N ) = WORK( N, N ) - D( N )
DO 50 I = 2, N - 1
WORK( I, I-1 ) = WORK( I, I-1 ) - DL( I-1 )
WORK( I, I ) = WORK( I, I ) - D( I )
WORK( I, I+1 ) = WORK( I, I+1 ) - DU( I )
50 CONTINUE
END IF
*
* Compute the 1-norm of the tridiagonal matrix A.
*
ANORM = SLANGT( '1', N, DL, D, DU )
*
* Compute the 1-norm of WORK, which is only guaranteed to be
* upper Hessenberg.
*
RESID = SLANHS( '1', N, WORK, LDWORK, RWORK )
*
* Compute norm(L*U - A) / (norm(A) * EPS)
*
IF( ANORM.LE.ZERO ) THEN
IF( RESID.NE.ZERO )
$ RESID = ONE / EPS
ELSE
RESID = ( RESID / ANORM ) / EPS
END IF
*
RETURN
*
* End of SGTT01
*
END
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