Contents

# ?sytri

Computes the inverse of a symmetric matrix using
U
*
D
*
U
T
or
L
*
D
*
L
T
Bunch-Kaufman factorization.

## Syntax

Include Files
• mkl.h
Description
The routine computes the inverse
inv(
A
)
of a symmetric matrix
A
. Before calling this routine, call
?sytrf
to factorize
A
.
Input Parameters
matrix_layout
Specifies whether matrix storage layout is row major (
LAPACK_ROW_MAJOR
) or column major (
LAPACK_COL_MAJOR
).
uplo
Must be
'U'
or
'L'
.
Indicates how the input matrix
A
has been factored:
If
uplo
=
'U'
, the array
a
stores the Bunch-Kaufman factorization
A
=
U*D*U
T
.
If
uplo
=
'L'
, the array
a
stores the Bunch-Kaufman factorization
A
=
L*D*L
T
.
n
The order of the matrix
A
;
n
0
.
a
a
(size max(1,
lda
*
n
))
contains the factorization of the matrix
A
, as returned by
?sytrf
.
lda
The leading dimension of
a
;
lda
max(1,
n
)
.
ipiv
Array, size at least
max(1,
n
)
.
The
ipiv
array, as returned by
?sytrf
.
Output Parameters
a
Overwritten by the
n
-by-
n
matrix
inv(
A
)
.
Return Values
This function returns a value
info
.
If
info
= 0, the execution is successful.
If
info
=
-i
, parameter
i
had an illegal value.
If
info
=
i
, the
i
-th diagonal element of
D
is zero,
D
is singular, and the inversion could not be completed.
Application Notes
The computed inverse
X
satisfies the following error bounds:
`|D*UT*PT*X*P*U - I| ≤ c(n)ε(|D||UT|PT|X|P|U| + |D||D-1|)`
for
uplo
=
'U'
, and
`|D*LT*PT*X*P*L - I| ≤ c(n)ε(|D||LT|PT|X|P|L| + |D||D-1|)`
for . Here
c
(
n
)
is a modest linear function of
n
, and
ε
is the machine precision;
I
denotes the identity matrix.
The total number of floating-point operations is approximately
(2/3)
n
3
for real flavors and
(8/3)
n
3
for complex flavors.

#### Product and Performance Information

1

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