## Developer Reference

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

# p?gehd2

Reduces a general matrix to upper Hessenberg form by an orthogonal/unitary similarity transformation (unblocked algorithm).

## Syntax

Description
The
p?gehd2
routine
reduces a real/complex general distributed matrix sub(
A
) to upper Hessenberg form
H
by an orthogonal/unitary similarity transformation:
Q'
*sub(
A
)*
Q
=
H
, where
sub(
A
) =
A
(
ia
+
n
-1 :
ia
+
n
-1,
ja
+
n
-1 :
ja
+
n
-1)
.
Input Parameters
n
(global)
INTEGER
.
The order of the distributed submatrix
A
.
(
n
0)
.
ilo
,
ihi
(global)
INTEGER
.
It is assumed that the matrix sub(
A
) is already upper triangular in rows
ia
:
ia
+
ilo
-2
and
ia
+
ihi
:
ia
+
n
-1
and columns
ja
:
ja
+
jlo
-2
and
ja
+
jhi
:
ja
+
n
-1
.
See
Application Notes
for further information.
If
n
0
,
1 ≤
ilo
ihi
n
; otherwise set
ilo
= 1
,
ihi
=
n
.
a
(local).
REAL
for
psgehd2
DOUBLE PRECISION
for
pdgehd2
COMPLEX
for
pcgehd2
COMPLEX*16
for
pzgehd2
.
Pointer into the local memory to an array of size
(
lld_a
,
LOC
c
(
ja
+
n
-1))
.
On entry, this array contains the local pieces of the
n
-by-
n
general distributed matrix sub(
A
) to be reduced.
ia
,
ja
(global)
INTEGER
.
The row and column indices in the global matrix
A
indicating the first row and the first column of sub(
A
), respectively.
desca
(global and local)
INTEGER
array of size
dlen_
. The array descriptor for the distributed matrix
A
.
work
(local).
REAL
for
psgehd2
DOUBLE PRECISION
for
pdgehd2
COMPLEX
for
pcgehd2
COMPLEX*16
for
pzgehd2
.
This is a workspace array of size
lwork
.
lwork
(local or global)
INTEGER
.
The size of the array
work
.
lwork
is local input and must be a