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SLAGTS(3F)							    SLAGTS(3F)


NAME    [Toc]    [Back]

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

SYNOPSIS    [Toc]    [Back]

     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    [Toc]    [Back]

     SLAGTS 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) = 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    [Toc]    [Back]

     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.






									Page 1






SLAGTS(3F)							    SLAGTS(3F)



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

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

     D	     (input) REAL array, dimension (N-2)
	     On	entry, D must contain the second super-diagonal	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 positive 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.
SLAGTS(3F)							    SLAGTS(3F)


NAME    [Toc]    [Back]

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

SYNOPSIS    [Toc]    [Back]

     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    [Toc]    [Back]

     SLAGTS 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) = 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    [Toc]    [Back]

     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.






									Page 1






SLAGTS(3F)							    SLAGTS(3F)



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

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

     D	     (input) REAL array, dimension (N-2)
	     On	entry, D must contain the second super-diagonal	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 positive 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.


									PPPPaaaaggggeeee 2222
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