# mkl_cspblas_dbsrsymv

Computes matrix - vector product of a sparse symmetrical matrix stored in the BSR format (3-arrays variation) with zero-based indexing.

## Syntax

Fortran:

call mkl_cspblas_dbsrsymv(uplo, m, lb, a, ia, ja, x, y)

C:

mkl_cspblas_dbsrsymv(&uplo, &m, &lb, a, ia, ja, x, y);

## Description

The mkl_cspblas_dbsrsymv routine performs a matrix-vector operation defined as
`y := A*x`

or
`y := A'*x,`

where:

x and y are vectors,

A is an upper or lower triangle of the symmetrical sparse matrix in the BSR format (3-array variation) with zero-based indexing, A' is the transpose of A.

### Note

This routine supports only zero-based indexing of the input arrays.

## Input Parameters

Parameter descriptions are common for all implemented interfaces with the exception of data types that refer here to the FORTRAN 77 standard types. Data types specific to the different interfaces are described in the section “Interfaces” below.

uplo

  CHARACTER*1. Specifies whether the upper or low triangle of the matrix A is used.

  If `uplo = 'U'` or 'u', then the upper triangle of the matrix A is used.

  If `uplo = 'L'` or 'l', then the low triangle of the matrix A is used.

m

  INTEGER. Number of block rows of the matrix A.

lb

  INTEGER. Size of the block in the matrix A.

a

  REAL*8. Array containing elements of non-zero blocks of the matrix A. Its length is equal to the number of non-zero blocks in the matrix A multiplied by `lb*lb`. Refer to values array description in BSR Format for more details.

ia

  INTEGER. Array of length `(m + 1)`, containing indices of block in the array a, such that `ia(i)` is the index in the array a of the first non-zero element from the row i. The value of the last element `ia(m + 1)` is equal to the number of non-zero blocks plus one. Refer to rowIndex array description in BSR Format for more details.

ja

  REAL*8. Array containing the column indices for each non-zero block in the matrix A.

  Its length is equal to the number of non-zero blocks of the matrix A. Refer to columns array description in BSR Format for more details.

x

  REAL*8.

  Array, DIMENSION `(m*lb)`.

  On entry, the array x must contain the vector x.

## Output Parameters

y

  REAL*8.

  Array, DIMENSION at least `(m*lb)`.

  On exit, the array y must contain the vector y.

## Interfaces

## FORTRAN 77:

SUBROUTINE mkl_cspblas_dbsrsymv(uplo, m, lb, a, ia, ja, x, y)

CHARACTER*1 uplo

INTEGER m, lb

INTEGER ia(*), ja(*)

REAL*8 a(*), x(*), y(*)

## C:

void mkl_cspblas_dbsrsymv(char *uplo, int *m, int *lb,

double *a, int *ia, int *ja, double *x, double *y);

Parent topic: Sparse BLAS Level 2 and Level 3 Routines.