---
name: "dla_syamv.f"
section: "3"
platform: "linux"
package: "lapack-doc"
version: "3.12.1-2"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/dla_syamv.f"
---

# dla\_syamv.f(3)

## NAME

SRC/dla\_syamv.f

## SYNOPSIS

### Functions/Subroutines

subroutine **dla\_syamv** (uplo, n, alpha, a, lda, x, incx, beta, y, incy)\
 **DLA\_SYAMV** computes a matrix-vector product using a symmetric indefinite matrix to calculate error bounds.

## Function/Subroutine Documentation

### subroutine dla\_syamv (integer uplo, integer n, double precision alpha, double precision, dimension( lda, \* ) a, integer lda, double precision, dimension( \* ) x, integer incx, double precision beta, double precision, dimension( \* ) y, integer incy)

**DLA\_SYAMV** computes a matrix-vector product using a symmetric indefinite matrix to calculate error bounds.

**Purpose:**

```
!>
!> DLA_SYAMV  performs the matrix-vector operation
!>
!>         y := alpha*abs(A)*abs(x) + beta*abs(y),
!>
!> where alpha and beta are scalars, x and y are vectors and A is an
!> n by n symmetric matrix.
!>
!> This function is primarily used in calculating error bounds.
!> To protect against underflow during evaluation, components in
!> the resulting vector are perturbed away from zero by (N+1)
!> times the underflow threshold.  To prevent unnecessarily large
!> errors for block-structure embedded in general matrices,
!>  zero components are not perturbed.  A zero
!> entry is considered  if all multiplications involved
!> in computing that entry have at least one zero multiplicand.
!>
```

**Parameters**

*UPLO*

```
!>          UPLO is INTEGER
!>           On entry, UPLO specifies whether the upper or lower
!>           triangular part of the array A is to be referenced as
!>           follows:
!>
!>              UPLO = BLAS_UPPER   Only the upper triangular part of A
!>                                  is to be referenced.
!>
!>              UPLO = BLAS_LOWER   Only the lower triangular part of A
!>                                  is to be referenced.
!>
!>           Unchanged on exit.
!>
```

*N*

```
!>          N is INTEGER
!>           On entry, N specifies the number of columns of the matrix A.
!>           N must be at least zero.
!>           Unchanged on exit.
!>
```

*ALPHA*

```
!>          ALPHA is DOUBLE PRECISION .
!>           On entry, ALPHA specifies the scalar alpha.
!>           Unchanged on exit.
!>
```

*A*

```
!>          A is DOUBLE PRECISION array, dimension ( LDA, n ).
!>           Before entry, the leading m by n part of the array A must
!>           contain the matrix of coefficients.
!>           Unchanged on exit.
!>
```

*LDA*

```
!>          LDA is INTEGER
!>           On entry, LDA specifies the first dimension of A as declared
!>           in the calling (sub) program. LDA must be at least
!>           max( 1, n ).
!>           Unchanged on exit.
!>
```

*X*

```
!>          X is DOUBLE PRECISION array, dimension
!>           ( 1 + ( n - 1 )*abs( INCX ) )
!>           Before entry, the incremented array X must contain the
!>           vector x.
!>           Unchanged on exit.
!>
```

*INCX*

```
!>          INCX is INTEGER
!>           On entry, INCX specifies the increment for the elements of
!>           X. INCX must not be zero.
!>           Unchanged on exit.
!>
```

*BETA*

```
!>          BETA is DOUBLE PRECISION .
!>           On entry, BETA specifies the scalar beta. When BETA is
!>           supplied as zero then Y need not be set on input.
!>           Unchanged on exit.
!>
```

*Y*

```
!>          Y is DOUBLE PRECISION array, dimension
!>           ( 1 + ( n - 1 )*abs( INCY ) )
!>           Before entry with BETA non-zero, the incremented array Y
!>           must contain the vector y. On exit, Y is overwritten by the
!>           updated vector y.
!>
```

*INCY*

```
!>          INCY is INTEGER
!>           On entry, INCY specifies the increment for the elements of
!>           Y. INCY must not be zero.
!>           Unchanged on exit.
!>
```

**Author**

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

**Further Details:**

```
!>
!>  Level 2 Blas routine.
!>
!>  -- Written on 22-October-1986.
!>     Jack Dongarra, Argonne National Lab.
!>     Jeremy Du Croz, Nag Central Office.
!>     Sven Hammarling, Nag Central Office.
!>     Richard Hanson, Sandia National Labs.
!>  -- Modified for the absolute-value product, April 2006
!>     Jason Riedy, UC Berkeley
!>
```

Definition at line **175** of file **dla\_syamv.f**.

## Author

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