Purpose
To compute the eigenvalues of a selected 2-by-2 diagonal block of an upper quasi-triangular matrix, to reduce the selected block to the standard form and to split the block in the case of real eigenvalues by constructing an orthogonal transformation UT. This transformation is applied to A (by similarity) and to another matrix U from the right.Specification
SUBROUTINE MB03QY( N, L, A, LDA, U, LDU, E1, E2, INFO )
C .. Scalar Arguments ..
INTEGER INFO, L, LDA, LDU, N
DOUBLE PRECISION E1, E2
C .. Array Arguments ..
DOUBLE PRECISION A(LDA,*), U(LDU,*)
Arguments
Input/Output Parameters
N (input) INTEGER
The order of the matrices A and UT. N >= 2.
L (input) INTEGER
Specifies the position of the block. 1 <= L < N.
A (input/output) DOUBLE PRECISION array, dimension (LDA,N)
On entry, the leading N-by-N part of this array must
contain the upper quasi-triangular matrix A whose
selected 2-by-2 diagonal block is to be processed.
On exit, the leading N-by-N part of this array contains
the upper quasi-triangular matrix A after its selected
block has been splitt and/or put in the LAPACK standard
form.
LDA INTEGER
The leading dimension of array A. LDA >= N.
U (input/output) DOUBLE PRECISION array, dimension (LDU,N)
On entry, the leading N-by-N part of this array must
contain a transformation matrix U.
On exit, the leading N-by-N part of this array contains
U*UT, where UT is the transformation matrix used to
split and/or standardize the selected block.
LDU INTEGER
The leading dimension of array U. LDU >= N.
E1, E2 (output) DOUBLE PRECISION
E1 and E2 contain either the real eigenvalues or the real
and positive imaginary parts, respectively, of the complex
eigenvalues of the selected 2-by-2 diagonal block of A.
Error Indicator
INFO INTEGER
= 0: successful exit;
< 0: if INFO = -i, the i-th argument had an illegal
value.
Method
Let A1 = ( A(L,L) A(L,L+1) )
( A(L+1,L) A(L+1,L+1) )
be the specified 2-by-2 diagonal block of matrix A.
If the eigenvalues of A1 are complex, then they are computed and
stored in E1 and E2, where the real part is stored in E1 and the
positive imaginary part in E2. The 2-by-2 block is reduced if
necessary to the standard form, such that A(L,L) = A(L+1,L+1), and
A(L,L+1) and A(L+1,L) have oposite signs. If the eigenvalues are
real, the 2-by-2 block is reduced to an upper triangular form such
that ABS(A(L,L)) >= ABS(A(L+1,L+1)).
In both cases, an orthogonal rotation U1' is constructed such that
U1'*A1*U1 has the appropriate form. Let UT be an extension of U1
to an N-by-N orthogonal matrix, using identity submatrices. Then A
is replaced by UT'*A*UT and the contents of array U is U * UT.
Further Comments
NoneExample
Program Text
NoneProgram Data
NoneProgram Results
None