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+ SUBROUTINE ZGTSV( N, NRHS, DL, D, DU, B, LDB, INFO )
+*
+* -- LAPACK routine (version 3.1) --
+* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
+* November 2006
+*
+* .. Scalar Arguments ..
+ INTEGER INFO, LDB, N, NRHS
+* ..
+* .. Array Arguments ..
+ COMPLEX*16 B( LDB, * ), D( * ), DL( * ), DU( * )
+* ..
+*
+* Purpose
+* =======
+*
+* ZGTSV solves the equation
+*
+* A*X = B,
+*
+* where A is an N-by-N tridiagonal matrix, by Gaussian elimination with
+* partial pivoting.
+*
+* Note that the equation A'*X = B may be solved by interchanging the
+* order of the arguments DU and DL.
+*
+* Arguments
+* =========
+*
+* N (input) INTEGER
+* The order of the matrix A. N >= 0.
+*
+* NRHS (input) INTEGER
+* The number of right hand sides, i.e., the number of columns
+* of the matrix B. NRHS >= 0.
+*
+* DL (input/output) COMPLEX*16 array, dimension (N-1)
+* On entry, DL must contain the (n-1) subdiagonal elements of
+* A.
+* On exit, DL is overwritten by the (n-2) elements of the
+* second superdiagonal of the upper triangular matrix U from
+* the LU factorization of A, in DL(1), ..., DL(n-2).
+*
+* D (input/output) COMPLEX*16 array, dimension (N)
+* On entry, D must contain the diagonal elements of A.
+* On exit, D is overwritten by the n diagonal elements of U.
+*
+* DU (input/output) COMPLEX*16 array, dimension (N-1)
+* On entry, DU must contain the (n-1) superdiagonal elements
+* of A.
+* On exit, DU is overwritten by the (n-1) elements of the first
+* superdiagonal of U.
+*
+* B (input/output) COMPLEX*16 array, dimension (LDB,NRHS)
+* On entry, the N-by-NRHS right hand side matrix B.
+* On exit, if INFO = 0, the N-by-NRHS solution matrix X.
+*
+* LDB (input) INTEGER
+* The leading dimension of the array B. LDB >= max(1,N).
+*
+* INFO (output) INTEGER
+* = 0: successful exit
+* < 0: if INFO = -i, the i-th argument had an illegal value
+* > 0: if INFO = i, U(i,i) is exactly zero, and the solution
+* has not been computed. The factorization has not been
+* completed unless i = N.
+*
+* =====================================================================
+*
+* .. Parameters ..
+ COMPLEX*16 ZERO
+ PARAMETER ( ZERO = ( 0.0D+0, 0.0D+0 ) )
+* ..
+* .. Local Scalars ..
+ INTEGER J, K
+ COMPLEX*16 MULT, TEMP, ZDUM
+* ..
+* .. Intrinsic Functions ..
+ INTRINSIC ABS, DBLE, DIMAG, MAX
+* ..
+* .. External Subroutines ..
+ EXTERNAL XERBLA
+* ..
+* .. Statement Functions ..
+ DOUBLE PRECISION CABS1
+* ..
+* .. Statement Function definitions ..
+ CABS1( ZDUM ) = ABS( DBLE( ZDUM ) ) + ABS( DIMAG( ZDUM ) )
+* ..
+* .. Executable Statements ..
+*
+ INFO = 0
+ IF( N.LT.0 ) THEN
+ INFO = -1
+ ELSE IF( NRHS.LT.0 ) THEN
+ INFO = -2
+ ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
+ INFO = -7
+ END IF
+ IF( INFO.NE.0 ) THEN
+ CALL XERBLA( 'ZGTSV ', -INFO )
+ RETURN
+ END IF
+*
+ IF( N.EQ.0 )
+ $ RETURN
+*
+ DO 30 K = 1, N - 1
+ IF( DL( K ).EQ.ZERO ) THEN
+*
+* Subdiagonal is zero, no elimination is required.
+*
+ IF( D( K ).EQ.ZERO ) THEN
+*
+* Diagonal is zero: set INFO = K and return; a unique
+* solution can not be found.
+*
+ INFO = K
+ RETURN
+ END IF
+ ELSE IF( CABS1( D( K ) ).GE.CABS1( DL( K ) ) ) THEN
+*
+* No row interchange required
+*
+ MULT = DL( K ) / D( K )
+ D( K+1 ) = D( K+1 ) - MULT*DU( K )
+ DO 10 J = 1, NRHS
+ B( K+1, J ) = B( K+1, J ) - MULT*B( K, J )
+ 10 CONTINUE
+ IF( K.LT.( N-1 ) )
+ $ DL( K ) = ZERO
+ ELSE
+*
+* Interchange rows K and K+1
+*
+ MULT = D( K ) / DL( K )
+ D( K ) = DL( K )
+ TEMP = D( K+1 )
+ D( K+1 ) = DU( K ) - MULT*TEMP
+ IF( K.LT.( N-1 ) ) THEN
+ DL( K ) = DU( K+1 )
+ DU( K+1 ) = -MULT*DL( K )
+ END IF
+ DU( K ) = TEMP
+ DO 20 J = 1, NRHS
+ TEMP = B( K, J )
+ B( K, J ) = B( K+1, J )
+ B( K+1, J ) = TEMP - MULT*B( K+1, J )
+ 20 CONTINUE
+ END IF
+ 30 CONTINUE
+ IF( D( N ).EQ.ZERO ) THEN
+ INFO = N
+ RETURN
+ END IF
+*
+* Back solve with the matrix U from the factorization.
+*
+ DO 50 J = 1, NRHS
+ B( N, J ) = B( N, J ) / D( N )
+ IF( N.GT.1 )
+ $ B( N-1, J ) = ( B( N-1, J )-DU( N-1 )*B( N, J ) ) / D( N-1 )
+ DO 40 K = N - 2, 1, -1
+ B( K, J ) = ( B( K, J )-DU( K )*B( K+1, J )-DL( K )*
+ $ B( K+2, J ) ) / D( K )
+ 40 CONTINUE
+ 50 CONTINUE
+*
+ RETURN
+*
+* End of ZGTSV
+*
+ END