Contents

# cblas_?tbmv

Computes a matrix-vector product using a triangular band matrix.

## Syntax

Include Files
• mkl.h
Description
The
?tbmv
routines perform one of the matrix-vector operations defined as
x
:=
A
*
x
, or
x
:=
A
'*
x
, or
x
:= conjg(
A
')*
x
,
where:
x
is an
n
-element vector,
A
is an
n
-by-
n
unit, or non-unit, upper or lower triangular band matrix, with
(
k
+1)
diagonals.
Input Parameters
Layout
Specifies whether two-dimensional array storage is row-major (
CblasRowMajor
) or column-major (
CblasColMajor
).
uplo
Specifies whether the matrix
A
is an upper or lower triangular matrix:
uplo
=
CblasUpper
if
uplo
=
CblasLower
, then the matrix is low triangular.
trans
Specifies the operation:
if
trans
=
CblasNoTrans
, then
x
:=
A
*
x
;
if
trans
=
CblasTrans
, then
x
:=
A
'*
x
;
if
trans
=
CblasConjTrans
, then
x
:= conjg(
A
')*
x
.
diag
Specifies whether the matrix
A
is unit triangular:
if
diag
=
CblasUnit
then the matrix is unit triangular;
if
diag
=
CblasNonUnit
, then the matrix is not unit triangular.
n
Specifies the order of the matrix
A
. The value of
n
must be at least zero.
k
On entry with
uplo
=
CblasUpper
specifies the number of super-diagonals of the matrix
A
. On entry with
uplo
=
CblasLower
,
k
specifies the number of sub-diagonals of the matrix
a
.
The value of
k
must satisfy
0
k
.
a
Array, size
lda
*
n
.
Layout
=
CblasColMajor
:
Before entry with
uplo
=
CblasUpper
, the leading
(
k
+ 1)
by
n
part of the array
a
must contain the upper triangular band part of the matrix of coefficients, supplied column-by-column,
with the leading diagonal of the matrix in row
k
of the array, the first super-diagonal starting at position 1 in row
(
k
- 1)
, and so on.
The top left
k
by
k
triangle of the array
a
is not referenced. The following program segment transfers an upper triangular band matrix from conventional full matrix storage (
matrix
, with leading dimension
ldm
) to band storage (
a
, with leading dimension
lda
):
```for (j = 0; j < n; j++) {
m = k - j;
for (i = max( 0, j - k); i <= j; i++) {
a[(m+i) + j*lda] = matrix[i + j*ldm];
}
}
```
Before entry with
uplo
=
CblasLower
, the leading
(
k
+ 1)
by
n
part of the array
a
must contain the lower triangular band part of the matrix of coefficients, supplied column-by-column,
with the leading diagonal of the matrix in row 0 of the array, the first sub-diagonal starting at position 0 in row 1
, and so on. The bottom right
k
by
k
triangle of the array
a
is not referenced. The following program segment transfers a lower triangular band matrix from conventional full matrix storage (
matrix
, with leading dimension
ldm
) to band storage (
a
, with leading dimension
lda
):
```for (j = 0; j < n; j++) {
m = -j;
for (i = j; i < min(n, j + k + 1); i++) {
a[(m+i) + j*lda] = matrix[i + j*ldm];
}
}```
Note that when
diag
=
CblasUnit
, the elements of the array
a
corresponding to the diagonal elements of the matrix are not referenced, but are assumed to be unity.
Layout = CblasRowMajor:
Before entry with
uplo
=
CblasUpper
, the leading (
k
+ 1)-by-
n
part of array
a
must contain the upper triangular band part of the matrix of coefficients. The matrix must be supplied row-by-row, with the leading diagonal of the matrix in column 0 of the array, the first super-diagonal starting at position 0 in column 1, and so on. The bottom right
k
-by-
k
triangle of array
a
is not referenced.
The following program segment transfers the upper triangular part of a Hermitian band matrix from row-major full matrix storage (
matrix
with leading dimension
ldm
) to row-major band storage (
a
, with leading dimension
lda
):
```for (i = 0; i < n; i++) {
m = -i;
for (j = i; j < MIN(n, i+k+1); j++) {
a[(m+j) + i*lda] = matrix[j + i*ldm];
}
}
```
Before entry with
uplo
=
CblasLower
, the leading (
k
+ 1)-by-
n
part of array
a
must contain the lower triangular band part of the matrix of coefficients, supplied row-by-row, with the leading diagonal of the matrix in column
k
of the array, the first sub-diagonal starting at position 1 in column
k
-1, and so on. The top left
k
-by-
k
triangle of array
a
is not referenced.
The following program segment transfers the lower triangular part of a Hermitian row-major band matrix from row-major full matrix storage (
matrix
, with leading dimension
ldm
) to row-major band storage (
a
, with leading dimension
lda
):
```for (i = 0; i < n; i++) {
m = k - i;
for (j = max(0, i-k); j <= i; j++) {
a[(m+j) + i*lda] = matrix[j + i*ldm];
}
}```
lda
Specifies the leading dimension of
a
as declared in the calling (sub)program. The value of
lda
must be at least
(
k
+ 1)
.
x
Array, size at least
(1 + (
n
- 1)*abs(
incx
))
. Before entry, the incremented array
x
must contain the
n
-element vector
x
.
incx
Specifies the increment for the elements of
x
.
The value of
incx
must not be zero.
Output Parameters
x
Overwritten with the transformed vector
x
.

#### Product and Performance Information

1

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