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RedHat 9 (Linux i386) - man page for dlagts (redhat section l)

DLAGTS(l)					)					DLAGTS(l)

NAME
       DLAGTS  - may be used to solve one of the systems of equations  (T - lambda*I)*x = y or (T
       - lambda*I)'*x = y,

SYNOPSIS
       SUBROUTINE DLAGTS( JOB, N, A, B, C, D, IN, Y, TOL, INFO )

	   INTEGER	  INFO, JOB, N

	   DOUBLE	  PRECISION TOL

	   INTEGER	  IN( * )

	   DOUBLE	  PRECISION A( * ), B( * ), C( * ), D( * ), Y( * )

PURPOSE
       DLAGTS may be used to solve one of the systems of equations (T - lambda*I)*x = y or  (T	-
       lambda*I)'*x = y, where T is an n by n tridiagonal matrix, for x, following the factoriza-
       tion of (T - lambda*I) as

	  (T - lambda*I) = P*L*U ,

       by routine DLAGTF. The choice of equation to be solved is controlled by the argument  JOB,
       and  in each case there is an option to perturb zero or very small diagonal elements of U,
       this option being intended for use in applications such as inverse iteration.

ARGUMENTS
       JOB     (input) INTEGER
	       Specifies the job to be performed by DLAGTS as follows:
	       =  1: The equations  (T - lambda*I)x = y  are to be solved, but diagonal  elements
	       of U are not to be perturbed.  = -1: The equations  (T - lambda*I)x = y	are to be
	       solved and, if overflow would otherwise occur, the diagonal elements of U  are  to
	       be  perturbed.  See argument TOL below.	=  2: The equations  (T - lambda*I)'x = y
	       are to be solved, but diagonal elements of U are not to be perturbed.  =  -2:  The
	       equations  (T - lambda*I)'x = y	are to be solved and, if overflow would otherwise
	       occur, the diagonal elements of U are to be perturbed. See argument TOL below.

       N       (input) INTEGER
	       The order of the matrix T.

       A       (input) DOUBLE PRECISION array, dimension (N)
	       On entry, A must contain the diagonal elements of U as returned from DLAGTF.

       B       (input) DOUBLE PRECISION array, dimension (N-1)
	       On entry, B must contain the first super-diagonal elements of U as  returned  from
	       DLAGTF.

       C       (input) DOUBLE PRECISION array, dimension (N-1)
	       On entry, C must contain the sub-diagonal elements of L as returned from DLAGTF.

       D       (input) DOUBLE PRECISION array, dimension (N-2)
	       On  entry, D must contain the second super-diagonal elements of U as returned from
	       DLAGTF.

       IN      (input) INTEGER array, dimension (N)
	       On entry, IN must contain details of the matrix P as returned from DLAGTF.

       Y       (input/output) DOUBLE PRECISION array, dimension (N)
	       On entry, the right hand side vector y.	On exit, Y is overwritten by the solution
	       vector x.

       TOL     (input/output) DOUBLE PRECISION
	       On  entry,  with  JOB .lt. 0, TOL should be the minimum perturbation to be made to
	       very small diagonal elements of	U.   TOL  should  normally  be	chosen	as  about
	       eps*norm(U),  where  eps is the relative machine precision, but if TOL is supplied
	       as non-positive, then it is reset to eps*max( abs( u(i,j) ) ).	If   JOB  .gt.	0
	       then TOL is not referenced.

	       On  exit, TOL is changed as described above, only if TOL is non-positive on entry.
	       Otherwise TOL is unchanged.

       INFO    (output) INTEGER
	       = 0   : successful exit
	       element of the solution vector x. This can only occur when JOB is supplied as pos-
	       itive  and  either  means  that a diagonal element of U is very small, or that the
	       elements of the right-hand side vector y are very large.

LAPACK version 3.0			   15 June 2000 				DLAGTS(l)


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