
dlasq2.f(3) LAPACK dlasq2.f(3)
NAME
dlasq2.f 
SYNOPSIS
Functions/Subroutines
subroutine dlasq2 (N, Z, INFO)
DLASQ2 computes all the eigenvalues of the symmetric positive definite tridiagonal
matrix associated with the qd Array Z to high relative accuracy. Used by sbdsqr and
sstegr.
Function/Subroutine Documentation
subroutine dlasq2 (integerN, double precision, dimension( * )Z, integerINFO)
DLASQ2 computes all the eigenvalues of the symmetric positive definite tridiagonal matrix
associated with the qd Array Z to high relative accuracy. Used by sbdsqr and sstegr.
Purpose:
DLASQ2 computes all the eigenvalues of the symmetric positive
definite tridiagonal matrix associated with the qd array Z to high
relative accuracy are computed to high relative accuracy, in the
absence of denormalization, underflow and overflow.
To see the relation of Z to the tridiagonal matrix, let L be a
unit lower bidiagonal matrix with subdiagonals Z(2,4,6,,..) and
let U be an upper bidiagonal matrix with 1's above and diagonal
Z(1,3,5,,..). The tridiagonal is L*U or, if you prefer, the
symmetric tridiagonal to which it is similar.
Note : DLASQ2 defines a logical variable, IEEE, which is true
on machines which follow ieee754 floatingpoint standard in their
handling of infinities and NaNs, and false otherwise. This variable
is passed to DLASQ3.
Parameters:
N
N is INTEGER
The number of rows and columns in the matrix. N >= 0.
Z
Z is DOUBLE PRECISION array, dimension ( 4*N )
On entry Z holds the qd array. On exit, entries 1 to N hold
the eigenvalues in decreasing order, Z( 2*N+1 ) holds the
trace, and Z( 2*N+2 ) holds the sum of the eigenvalues. If
N > 2, then Z( 2*N+3 ) holds the iteration count, Z( 2*N+4 )
holds NDIVS/NIN^2, and Z( 2*N+5 ) holds the percentage of
shifts that failed.
INFO
INFO is INTEGER
= 0: successful exit
< 0: if the ith argument is a scalar and had an illegal
value, then INFO = i, if the ith argument is an
array and the jentry had an illegal value, then
INFO = (i*100+j)
> 0: the algorithm failed
= 1, a split was marked by a positive value in E
= 2, current block of Z not diagonalized after 100*N
iterations (in inner while loop). On exit Z holds
a qd array with the same eigenvalues as the given Z.
= 3, termination criterion of outer while loop not met
(program created more than N unreduced blocks)
Author:
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Date:
September 2012
Further Details:
Local Variables: I0:N0 defines a current unreduced segment of Z.
The shifts are accumulated in SIGMA. Iteration count is in ITER.
Pingpong is controlled by PP (alternates between 0 and 1).
Definition at line 113 of file dlasq2.f.
Author
Generated automatically by Doxygen for LAPACK from the source code.
Version 3.4.2 Tue Sep 25 2012 dlasq2.f(3) 
