Provided by: liblapack-doc_3.3.1-1_all NAME

LAPACK-3  -  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

SYNOPSIS

SUBROUTINE DLASQ2( N, Z, INFO )

INTEGER        INFO, N

DOUBLE         PRECISION Z( * )

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 ieee-754 floating-point standard in their
handling of infinities and NaNs, and false otherwise. This variable
is passed to DLASQ3.

ARGUMENTS

N     (input) INTEGER
The number of rows and columns in the matrix. N >= 0.

Z     (input/output) 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  (output) INTEGER
= 0: successful exit
< 0: if the i-th argument is a scalar and had an illegal
value, then INFO = -i, if the i-th argument is an
array and the j-entry 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 30*N
iterations (in inner while loop)
= 3, termination criterion of outer while loop not met
(program created more than N unreduced blocks)

FURTHERDETAILS

The shifts are accumulated in SIGMA. Iteration count is in ITER.
Ping-pong is controlled by PP (alternates between 0 and 1).

LAPACK routine (version 3.2)               April 2011                            DLASQ2(3lapack)