##### syr2

###### Prototype

There is one prototype of `syr2` available, please see below.

    syr2( const Scalar >, const VectorX& x, const VectorY& y, MatrixA& a );

###### Description

`syr2` (short for symmetric rank-2 update) provides a C++ interface to BLAS routines SSYR2 and DSYR2. `syr2` performs the symmetric rank 2 operation

A := alpha*x*y' + alpha*y*x' + A,

where alpha is a scalar, x and y are n element vectors and A is an n by n symmetric matrix.

The selection of the BLAS routine is done during compile-time, and is determined by the type of values contained in type `VectorX`. The type of values is obtained through the `value_type` meta-function `typename value_type<VectorX>::type`. Table X below illustrates to which specific routine this dispatching will take place.

Table 1.66. Dispatching of syr2.

Value type of VectorX | BLAS routine | CBLAS routine | CUBLAS routine
--- | --- | --- | ---
`float` | SSYR2 | cblas_ssyr2 | cublasSsyr2
`double` | DSYR2 | cblas_dsyr2 | Unavailable

The original routines SSYR2 and DSYR2 have nine arguments, whereas `syr2` requires four arguments.

Table 1.67. Deduction of arguments of syr2.

###### Definition

Defined in header `boost/numeric/bindings/blas/level2/syr2.hpp`.

###### Parameters or Requirements on Types

Parameters

MatrixA

  The definition of term 1

MatrixB

  The definition of term 2

MatrixC

  The definition of term 3.

  Definitions may contain paragraphs.

###### Complexity

###### Example

    #include <boost/numeric/bindings/blas/level2/syr2.hpp>
    using namespace boost::numeric::bindings;

    blas::syr2( x, y, z );

this will output

    [5] 0 1 2 3 4 5

###### Notes

###### See Also

- Originating Fortran source files ssyr2.f and dsyr2.f at Netlib.