nag_dsymv (f16pcc) (PDF version)
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f16 Chapter Introduction
NAG C Library Manual

NAG Library Function Document

nag_dsymv (f16pcc)

+ Contents

    1  Purpose
    7  Accuracy

1  Purpose

nag_dsymv (f16pcc) performs matrix-vector multiplication for a real symmetric matrix.

2  Specification

#include <nag.h>
#include <nagf16.h>
void  nag_dsymv (Nag_OrderType order, Nag_UploType uplo, Integer n, double alpha, const double a[], Integer pda, const double x[], Integer incx, double beta, double y[], Integer incy, NagError *fail)

3  Description

nag_dsymv (f16pcc) performs the matrix-vector operation
yαAx+βy,
where A is an n by n real symmetric matrix, x and y are n-element real vectors, and α and β are real scalars.

4  References

The BLAS Technical Forum Standard (2001) http://www.netlib.org/blas/blast-forum

5  Arguments

1:     orderNag_OrderTypeInput
On entry: the order argument specifies the two-dimensional storage scheme being used, i.e., row-major ordering or column-major ordering. C language defined storage is specified by order=Nag_RowMajor. See Section 3.2.1.3 in the Essential Introduction for a more detailed explanation of the use of this argument.
Constraint: order=Nag_RowMajor or Nag_ColMajor.
2:     uploNag_UploTypeInput
On entry: specifies whether the upper or lower triangular part of A is stored.
uplo=Nag_Upper
The upper triangular part of A is stored.
uplo=Nag_Lower
The lower triangular part of A is stored.
Constraint: uplo=Nag_Upper or Nag_Lower.
3:     nIntegerInput
On entry: n, the order of the matrix A.
Constraint: n0.
4:     alphadoubleInput
On entry: the scalar α.
5:     a[dim]const doubleInput
Note: the dimension, dim, of the array a must be at least max1,pda×n.
On entry: the n by n symmetric matrix A.
If order=Nag_ColMajor, Aij is stored in a[j-1×pda+i-1].
If order=Nag_RowMajor, Aij is stored in a[i-1×pda+j-1].
If uplo=Nag_Upper, the upper triangular part of A must be stored and the elements of the array below the diagonal are not referenced.
If uplo=Nag_Lower, the lower triangular part of A must be stored and the elements of the array above the diagonal are not referenced.
6:     pdaIntegerInput
On entry: the stride separating row or column elements (depending on the value of order) of the matrix A in the array a.
Constraint: pdamax1,n.
7:     x[dim]const doubleInput
Note: the dimension, dim, of the array x must be at least max1,1+n-1incx.
On entry: the vector x.
8:     incxIntegerInput
On entry: the increment in the subscripts of x between successive elements of x.
Constraint: incx0.
9:     betadoubleInput
On entry: the scalar β.
10:   y[dim]doubleInput/Output
Note: the dimension, dim, of the array y must be at least max1,1+n-1incy.
On entry: the vector y.
If beta=0, y need not be set.
On exit: the updated vector y.
11:   incyIntegerInput
On entry: the increment in the subscripts of y between successive elements of y.
Constraint: incy0.
12:   failNagError *Input/Output
The NAG error argument (see Section 3.6 in the Essential Introduction).

6  Error Indicators and Warnings

NE_BAD_PARAM
On entry, argument value had an illegal value.
NE_INT
On entry, incx=value.
Constraint: incx0.
On entry, incy=value.
Constraint: incy0.
On entry, n=value.
Constraint: n0.
NE_INT_2
On entry, pda=value, n=value.
Constraint: pdamax1,n.

7  Accuracy

The BLAS standard requires accurate implementations which avoid unnecessary over/underflow (see Section 2.7 of The BLAS Technical Forum Standard (2001)).

8  Further Comments

None.

9  Example

This example computes the matrix-vector product
y=αAx+βy
where
A = 1.0 2.0 3.0 2.0 4.0 5.0 3.0 5.0 6.0 ,
x = -1.0 2.0 -3.0 ,
y = 1.0 2.0 3.0 ,
α=1.5   and   ​ β=1.0 .

9.1  Program Text

Program Text (f16pcce.c)

9.2  Program Data

Program Data (f16pcce.d)

9.3  Program Results

Program Results (f16pcce.r)


nag_dsymv (f16pcc) (PDF version)
f16 Chapter Contents
f16 Chapter Introduction
NAG C Library Manual

© The Numerical Algorithms Group Ltd, Oxford, UK. 2012