dlasd5.f(3)

NAME
dlasd5.f -

SYNOPSIS

  Functions/Subroutines

subroutine dlasd5 (I, D, Z, DELTA, RHO, DSIGMA, WORK)
DLASD5 computes the square root of the i-th eigenvalue of a positive symmetric rank-one modification of a 2-by-2 diagonal matrix. Used by sbdsdc.

Function/Subroutine Documentation

  subroutine dlasd5 (integerI, double precision, dimension( 2 )D, double precision, dimension( 2 )Z, double precision, dimension( 2 )DELTA, double precisionRHO, double precisionDSIGMA, double precision, dimension( 2 )WORK)

DLASD5 computes the square root of the i-th eigenvalue of a positive symmetric rank-one modification of a 2-by-2 diagonal matrix. Used by sbdsdc.

Purpose:

     This subroutine computes the square root of the I-th eigenvalue
     of a positive symmetric rank-one modification of a 2-by-2 diagonal
     matrix

                diag( D ) * diag( D ) +  RHO * Z * transpose(Z) .

     The diagonal entries in the array D are assumed to satisfy

                0 <= D(i) < D(j)  for  i < j .

     We also assume RHO > 0 and that the Euclidean norm of the vector
     Z is one.

Parameters:
    I

              I is INTEGER
             The index of the eigenvalue to be computed.  I = 1 or I = 2.

    D

              D is DOUBLE PRECISION array, dimension ( 2 )
             The original eigenvalues.  We assume 0 <= D(1) < D(2).

    Z

              Z is DOUBLE PRECISION array, dimension ( 2 )
             The components of the updating vector.

    DELTA

              DELTA is DOUBLE PRECISION array, dimension ( 2 )
             Contains (D(j) - sigma_I) in its  j-th component.
             The vector DELTA contains the information necessary
             to construct the eigenvectors.

    RHO

              RHO is DOUBLE PRECISION
             The scalar in the symmetric updating formula.

    DSIGMA

              DSIGMA is DOUBLE PRECISION
             The computed sigma_I, the I-th updated eigenvalue.

    WORK

              WORK is DOUBLE PRECISION array, dimension ( 2 )
             WORK contains (D(j) + sigma_I) in its  j-th component.

Author:
    Univ. of Tennessee

    Univ. of California Berkeley

    Univ. of Colorado Denver

    NAG Ltd.

Date:
    September 2012

Contributors:
    Ren-Cang Li, Computer Science Division, University of California at Berkeley, USA

Definition at line 117 of file dlasd5.f.

Author

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