
SLAGTS(l) ) SLAGTS(l)
NAME
SLAGTS  may be used to solve one of the systems of equations (T  lambda*I)*x = y or (T
 lambda*I)'*x = y,
SYNOPSIS
SUBROUTINE SLAGTS( JOB, N, A, B, C, D, IN, Y, TOL, INFO )
INTEGER INFO, JOB, N
REAL TOL
INTEGER IN( * )
REAL A( * ), B( * ), C( * ), D( * ), Y( * )
PURPOSE
SLAGTS 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 SLAGTF. 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 SLAGTS 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) REAL array, dimension (N)
On entry, A must contain the diagonal elements of U as returned from SLAGTF.
B (input) REAL array, dimension (N1)
On entry, B must contain the first superdiagonal elements of U as returned from
SLAGTF.
C (input) REAL array, dimension (N1)
On entry, C must contain the subdiagonal elements of L as returned from SLAGTF.
D (input) REAL array, dimension (N2)
On entry, D must contain the second superdiagonal elements of U as returned from
SLAGTF.
IN (input) INTEGER array, dimension (N)
On entry, IN must contain details of the matrix P as returned from SLAGTF.
Y (input/output) REAL array, dimension (N)
On entry, the right hand side vector y. On exit, Y is overwritten by the solution
vector x.
TOL (input/output) REAL
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 nonpositive, 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 nonpositive 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 righthand side vector y are very large.
LAPACK version 3.0 15 June 2000 SLAGTS(l) 
