## Developer Reference

• 2020.2
• 07/15/2020
• Public Content
Contents

# ?larft

Forms the triangular factor
T
of a block reflector
H
=
I
-
V
*
T
*
V
**
H
.

## Syntax

Include Files
• mkl.fi
Description
The routine
?larft
forms the triangular factor
T
of a real/complex block reflector
H
of order
n
, which is defined as a product of
k
elementary reflectors.
If
direct
=
'F'
,
H
=
H
(1)*
H
(2)* . . .*
H
(
k
)
and
T
is upper triangular;
If
direct
=
'B'
,
H
=
H
(
k
)*. . .*
H
(2)*
H
(1)
and
T
is lower triangular.
If
storev
=
'C'
, the vector which defines the elementary reflector H(
i
) is stored in the
i
-th column of the array
v
, and
H
=
I
-
V
*
T
*
V
T
(for real flavors) or
H
=
I
-
V
*
T
*
V
H
(for complex flavors) .
If
storev
=
'R'
, the vector which defines the elementary reflector H(
i
) is stored in the
i
-th row of the array
v
, and
H
=
I
-
V
T
*
T
*
V
(for real flavors) or
H
=
I
-
V
H
*
T
*
V
(for complex flavors).
Input Parameters
The data types are given for the Fortran interface.
direct
CHARACTER*1
.
Specifies the order in which the elementary reflectors are multiplied to form the block reflector:
=
'F'
:
H
=
H
(1)*
H
(2)*. . . *
H
(
k
)
(forward)
=
'B'
:
H
=
H
(
k
)*. . .*
H
(2)*
H
(1)
(backward)
storev
CHARACTER*1
.
Specifies how the vectors which define the elementary reflectors are stored
Application Notes
below)
:
=
'C'
: column-wise
=
'R'
: row-wise.
INTEGER
.
The order of the block reflector
H
.
n
0
.
k
INTEGER
.
The order of the triangular factor
T
(equal to the number of elementary reflectors).
k
1
.
v
REAL
for
slarft
DOUBLE PRECISION
for
dlarft
COMPLEX
for
clarft
DOUBLE COMPLEX
for
zlarft
Array,
DIMENSION
(
ldv
,
k
) if