SRC/slagtf.f(3)

NAME
SRC/slagtf.f

SYNOPSIS

  Functions/Subroutines
subroutine slagtf (n, a, lambda, b, c, tol, d, in, info)
SLAGTF computes an LU factorization of a matrix T-λI, where T is a general tridiagonal matrix, and λ a scalar, using partial pivoting with row interchanges.

Function/Subroutine Documentation

  subroutine slagtf (integer n, real, dimension( * ) a, real lambda, real, dimension( * ) b, real, dimension( * ) c, real tol, real, dimension( * ) d, integer, dimension( * ) in, integer info)

SLAGTF computes an LU factorization of a matrix T-λI, where T is a general tridiagonal matrix, and λ a scalar, using partial pivoting with row interchanges.

Purpose:

     SLAGTF factorizes the matrix (T - lambda*I), where T is an n by n
     tridiagonal matrix and lambda is a scalar, as
        T - lambda*I = PLU,
     where P is a permutation matrix, L is a unit lower tridiagonal matrix
     with at most one non-zero sub-diagonal elements per column and U is
     an upper triangular matrix with at most two non-zero super-diagonal
     elements per column.
     The factorization is obtained by Gaussian elimination with partial
     pivoting and implicit row scaling.
     The parameter LAMBDA is included in the routine so that SLAGTF may
     be used, in conjunction with SLAGTS, to obtain eigenvectors of T by
     inverse iteration.

Parameters
    N

              N is INTEGER
              The order of the matrix T.

    A

              A is REAL array, dimension (N)
              On entry, A must contain the diagonal elements of T.
              On exit, A is overwritten by the n diagonal elements of the
              upper triangular matrix U of the factorization of T.

    LAMBDA

              LAMBDA is REAL
              On entry, the scalar lambda.

    B

              B is REAL array, dimension (N-1)
              On entry, B must contain the (n-1) super-diagonal elements of
              T.
              On exit, B is overwritten by the (n-1) super-diagonal
              elements of the matrix U of the factorization of T.

    C

              C is REAL array, dimension (N-1)
              On entry, C must contain the (n-1) sub-diagonal elements of
              T.
              On exit, C is overwritten by the (n-1) sub-diagonal elements
              of the matrix L of the factorization of T.

    TOL

              TOL is REAL
              On entry, a relative tolerance used to indicate whether or
              not the matrix (T - lambda*I) is nearly singular. TOL should
              normally be chose as approximately the largest relative error
              in the elements of T. For example, if the elements of T are
              correct to about 4 significant figures, then TOL should be
              set to about 5*10**(-4). If TOL is supplied as less than eps,
              where eps is the relative machine precision, then the value
              eps is used in place of TOL.

    D

              D is REAL array, dimension (N-2)
              On exit, D is overwritten by the (n-2) second super-diagonal
              elements of the matrix U of the factorization of T.

    IN

              IN is INTEGER array, dimension (N)
              On exit, IN contains details of the permutation matrix P. If
              an interchange occurred at the kth step of the elimination,
              then IN(k) = 1, otherwise IN(k) = 0. The element IN(n)
              returns the smallest positive integer j such that
                 abs( u(j,j) ) <= norm( (T - lambda*I)(j) )*TOL,
              where norm( A(j) ) denotes the sum of the absolute values of
              the jth row of the matrix A. If no such j exists then IN(n)
              is returned as zero. If IN(n) is returned as positive, then a
              diagonal element of U is small, indicating that
              (T - lambda*I) is singular or nearly singular,

    INFO

              INFO is INTEGER
              = 0: successful exit
              < 0: if INFO = -k, the kth argument had an illegal value

Author
    Univ. of Tennessee

    Univ. of California Berkeley

    Univ. of Colorado Denver

    NAG Ltd.

Definition at line 155 of file slagtf.f.

Author

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