Contents

# sytrf

Computes the Bunch-Kaufman factorization of a symmetric matrix. This routine belongs to the
oneapi::mkl::lapack
namespace.

## Description

The routine computes the factorization of a real/complex symmetric matrix
A
using the Bunch-Kaufman diagonal pivoting method. The form of the factorization is:
• if
uplo=uplo::upper
,
A
=
U*D*U
T
• if
uplo=uplo::lower
,
A
=
L*D*L
T
where
A
is the input matrix,
U
and
L
are products of permutation and triangular matrices with unit diagonal (upper triangular for
U
and lower triangular for
L
), and
D
is a symmetric block-diagonal matrix with 1-by-1 and 2-by-2 diagonal blocks.
U
and
L
have 2-by-2 unit diagonal blocks corresponding to the 2-by-2 blocks of
D
.

## API

Syntax
``````namespace oneapi::mkl::lapack {
void sytrf(cl::sycl::queue &queue,
mkl::uplo uplo,
std::int64_t n,
cl::sycl::buffer<T> &a,
std::int64_t lda,
std::int64_t *ipiv,
}``````
sytrf
supports the following precisions and devices:
T
Devices Supported
float
Host and CPU
double
Host and CPU
std::complex<float>
Host and CPU
std::complex<double>
Host and CPU
Input Parameters
 queue Device queue where calculations will be performed. uplo Indicates whether the upper or lower triangular part of A is stored and how A is factored: If uplo=uplo::upper, the buffer a stores the upper triangular part of the matrix A, and A is factored as U*D*UT. If uplo=uplo::lower, the buffer a stores the lower triangular part of the matrix A, and A is factored as L*D*LT. n The order of matrix A (0≤n). a Buffer holding the coefficients of matrix A, size max(1,lda*n) containing either the upper or lower triangular part of the matrix A (see uplo). The second dimension of a must be at least max(1, n). lda The leading dimension of a. scratchpad Buffer holding scratchpad memory to be used by the routine for storing intermediate results. scratchpad_size Size of scratchpad memory as a number of floating point elements of type T. Size should not be less than the value returned by the sytrf_scratchpad_size function.
Output Parameters
 a The upper or lower triangular part of a is overwritten by details of the block-diagonal matrix D and the multipliers used to obtain the factor U (or L). ipiv Buffer holding array of size at least max(1, n). Contains details of the interchanges and the block structure of D. If ipiv(i)=k>0, then dii is a 1-by-1 block, and the i-th row and column of A was interchanged with the k-th row and column. If uplo=mkl::uplo::upper and ipiv(i)=ipiv(i-1)=-m<0, then D has a 2-by-2 block in rows/columns i and i-1, and (i-1)-th row and column of A was interchanged with the m-th row and column. If uplo=mkl::uplo::lower and ipiv(i)=ipiv(i+1)=-m<0, then D has a 2-by-2 block in rows/columns i and i+1, and (i+1)-th row and column of A was interchanged with the m-th row and column.
Exceptions
Exception
Description
mkl::lapack::exception
This exception is thrown when problems occur during calculations. You can obtain the info code of the problem using the info() method of the exception object:
If
info = -i
, the
i
-th parameter had an illegal value.
If
info = i
,
dii
is 0. The factorization has been completed, but
D
is exactly singular. Division by 0 will occur if you use
D
for solving a system of linear equations.
If
info
is equal to the value passed as scratchpad size, and detail() returns non zero, then the passed scratchpad has an insufficient size, and the required size should not be less than the value returned by the detail() method of the exception object.

#### Product and Performance Information

1

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