Contents

# ?ormhr

Multiplies an arbitrary real matrix C by the real orthogonal matrix Q determined by
?gehrd
.

## Syntax

Include Files
• mkl.h
Description
The routine multiplies a matrix
C
by the orthogonal matrix
Q
that has been determined by a preceding call to
sgehrd
/
dgehrd
. (The routine
?gehrd
reduces a real general matrix
A
to upper Hessenberg form
H
by an orthogonal similarity transformation,
A
=
Q*H*Q
T
, and represents the matrix
Q
as a product of
ihi
-
ilo
elementary reflectors. Here
ilo
and
ihi
are values determined by
sgebal
/
dgebal
when balancing the matrix;if the matrix has not been balanced,
ilo
= 1
and
ihi
=
n
.)
With
?ormhr
, you can form one of the matrix products
Q
*
C
,
Q
T
*
C
,
C
*
Q
, or
C
*
Q
T
, overwriting the result on
C
(which may be any real rectangular matrix).
A common application of
?ormhr
is to transform a matrix
V
of eigenvectors of
H
to the matrix
QV
of eigenvectors of
A
.
Input Parameters
matrix_layout
Specifies whether matrix storage layout is row major (
LAPACK_ROW_MAJOR
) or column major (
LAPACK_COL_MAJOR
).
side
Must be
'L'
or
'R'
.
If
side
= 'L'
, then the routine forms
Q
*
C
or
Q
T
*
C
.
If
side
= 'R'
, then the routine forms
C
*
Q
or
C
*
Q
T
.
trans
Must be
'N'
or
'T'
.
If
trans
= 'N'
, then
Q
is applied to
C
.
If
trans
= 'T'
, then
Q
T
is applied to
C
.
m
The number of rows in
C
(
m
0
).
n
The number of columns in
C
(
n
0
).
ilo
,
ihi
These must be the same parameters
ilo
and
ihi
, respectively, as supplied to
?gehrd
.
If
m
> 0
and
side
=
'L'
, then
1
ilo
ihi
m
.
If
m
= 0
and
side
=
'L'
, then
ilo
= 1
and
ihi
= 0
.
If
n
> 0
and
side
=
'R'
, then
1
ilo
ihi
n
.
If
n
= 0
and
side
=
'R'
, then
ilo
= 1
and
ihi
= 0
.
a
,
tau
,
c
Arrays:
a
(size max(1,
lda
*
n
) for
side
='R' and size max(1,
lda
*
m
) for
side
='L')
contains details of the vectors which define the elementary reflectors, as returned by
?gehrd
.
tau
contains further details of the elementary reflectors, as returned by
?gehrd
.
The dimension of
tau
must be at least max (1,
m
-1) if
side
=
'L'
and at least max (1,
n
-1) if
side
=
'R'
.
c
(size max(1,
ldc
*
n
) for column major layout and max(1,
ldc
*
m
for row major layout)
contains the
m
by
n
matrix
C
.
lda
a
; at least max(1,
m
) if
side
=
'L'
and at least max (1,
n
) if
side
=
'R'
.
ldc
c
; at least max(1,
m
)
for column major layout and at least max(1,
n
) for row major layout
.
Output Parameters
c
C
is overwritten by product
Q
*
C
,
Q
T
*
C
,
C
*
Q
, or
C
*
Q
T
as specified by
side
and
trans
.
Return Values
This function returns a value
info
.
If
info
=0
, the execution is successful.
If
info
=
-i
, the
i
-th parameter had an illegal value.
Application Notes
The computed matrix
Q
differs from the exact result by a matrix
E
such that
||
E
||
2
=
O
(
ε
)|*|
C
||
2
, where
ε
is the machine precision.
The approximate number of floating-point operations is
2
n
(
ihi
-
ilo
)
2
if
side
=
'L'
;
2
m
(
ihi
-
ilo
)
2
if
side
=
'R'
.
The complex counterpart of this routine is unmhr.

#### Product and Performance Information

1

Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex.