dlagts (l)  Linux Man Pages
dlagts: may be used to solve one of the systems of equations (T  lambda*I)*x = y or (T  lambda*I)aq*x = y,
NAME
DLAGTS  may be used to solve one of the systems of equations (T  lambda*I)*x = y or (T  lambda*I)aq*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 where T is an n by n tridiagonal matrix, for x, following the factorization of (T  lambda*I) as(T  lambda*I)
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)aqx = y are to be solved, but diagonal elements of U are not to be perturbed. = 2: The equations (T  lambda*I)aqx = 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 (N1)
 On entry, B must contain the first superdiagonal elements of U as returned from DLAGTF.
 C (input) DOUBLE PRECISION array, dimension (N1)
 On entry, C must contain the subdiagonal elements of L as returned from DLAGTF.
 D (input) DOUBLE PRECISION array, dimension (N2)
 On entry, D must contain the second superdiagonal 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 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 positive 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.