SUBROUTINE SPTSV( N, NRHS, D, E, 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 .. REAL B( LDB, * ), D( * ), E( * ) * .. * * Purpose * ======= * * SPTSV computes the solution to a real system of linear equations * A*X = B, where A is an N-by-N symmetric positive definite tridiagonal * matrix, and X and B are N-by-NRHS matrices. * * A is factored as A = L*D*L**T, and the factored form of A is then * used to solve the system of equations. * * 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. * * D (input/output) REAL array, dimension (N) * On entry, the n diagonal elements of the tridiagonal matrix * A. On exit, the n diagonal elements of the diagonal matrix * D from the factorization A = L*D*L**T. * * E (input/output) REAL array, dimension (N-1) * On entry, the (n-1) subdiagonal elements of the tridiagonal * matrix A. On exit, the (n-1) subdiagonal elements of the * unit bidiagonal factor L from the L*D*L**T factorization of * A. (E can also be regarded as the superdiagonal of the unit * bidiagonal factor U from the U**T*D*U factorization of A.) * * B (input/output) REAL 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, the leading minor of order i is not * positive definite, and the solution has not been * computed. The factorization has not been completed * unless i = N. * * ===================================================================== * * .. External Subroutines .. EXTERNAL SPTTRF, SPTTRS, XERBLA * .. * .. Intrinsic Functions .. INTRINSIC MAX * .. * .. Executable Statements .. * * Test the input parameters. * 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 = -6 END IF IF( INFO.NE.0 ) THEN CALL XERBLA( 'SPTSV ', -INFO ) RETURN END IF * * Compute the L*D*L' (or U'*D*U) factorization of A. * CALL SPTTRF( N, D, E, INFO ) IF( INFO.EQ.0 ) THEN * * Solve the system A*X = B, overwriting B with X. * CALL SPTTRS( N, NRHS, D, E, B, LDB, INFO ) END IF RETURN * * End of SPTSV * END