100 lines
3.0 KiB
FortranFixed
100 lines
3.0 KiB
FortranFixed
SUBROUTINE SPTSV( N, NRHS, D, E, B, LDB, INFO )
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*
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* -- LAPACK routine (version 3.1) --
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* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
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* November 2006
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*
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* .. Scalar Arguments ..
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INTEGER INFO, LDB, N, NRHS
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* ..
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* .. Array Arguments ..
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REAL B( LDB, * ), D( * ), E( * )
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* ..
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*
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* Purpose
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* =======
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*
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* SPTSV computes the solution to a real system of linear equations
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* A*X = B, where A is an N-by-N symmetric positive definite tridiagonal
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* matrix, and X and B are N-by-NRHS matrices.
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*
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* A is factored as A = L*D*L**T, and the factored form of A is then
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* used to solve the system of equations.
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*
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* Arguments
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* =========
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*
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* N (input) INTEGER
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* The order of the matrix A. N >= 0.
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*
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* NRHS (input) INTEGER
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* The number of right hand sides, i.e., the number of columns
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* of the matrix B. NRHS >= 0.
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*
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* D (input/output) REAL array, dimension (N)
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* On entry, the n diagonal elements of the tridiagonal matrix
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* A. On exit, the n diagonal elements of the diagonal matrix
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* D from the factorization A = L*D*L**T.
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*
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* E (input/output) REAL array, dimension (N-1)
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* On entry, the (n-1) subdiagonal elements of the tridiagonal
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* matrix A. On exit, the (n-1) subdiagonal elements of the
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* unit bidiagonal factor L from the L*D*L**T factorization of
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* A. (E can also be regarded as the superdiagonal of the unit
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* bidiagonal factor U from the U**T*D*U factorization of A.)
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*
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* B (input/output) REAL array, dimension (LDB,NRHS)
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* On entry, the N-by-NRHS right hand side matrix B.
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* On exit, if INFO = 0, the N-by-NRHS solution matrix X.
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*
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* LDB (input) INTEGER
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* The leading dimension of the array B. LDB >= max(1,N).
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*
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* INFO (output) INTEGER
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* = 0: successful exit
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* < 0: if INFO = -i, the i-th argument had an illegal value
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* > 0: if INFO = i, the leading minor of order i is not
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* positive definite, and the solution has not been
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* computed. The factorization has not been completed
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* unless i = N.
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*
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* =====================================================================
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*
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* .. External Subroutines ..
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EXTERNAL SPTTRF, SPTTRS, XERBLA
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* ..
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* .. Intrinsic Functions ..
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INTRINSIC MAX
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* ..
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* .. Executable Statements ..
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*
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* Test the input parameters.
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*
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INFO = 0
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IF( N.LT.0 ) THEN
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INFO = -1
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ELSE IF( NRHS.LT.0 ) THEN
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INFO = -2
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ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
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INFO = -6
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END IF
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IF( INFO.NE.0 ) THEN
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CALL XERBLA( 'SPTSV ', -INFO )
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RETURN
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END IF
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*
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* Compute the L*D*L' (or U'*D*U) factorization of A.
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*
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CALL SPTTRF( N, D, E, INFO )
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IF( INFO.EQ.0 ) THEN
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*
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* Solve the system A*X = B, overwriting B with X.
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*
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CALL SPTTRS( N, NRHS, D, E, B, LDB, INFO )
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END IF
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RETURN
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*
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* End of SPTSV
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*
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END
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