Re: [Help-gsl] Vector-Matrix Multiplication

[Dimitris K.]

Indeed, I should have guessed that BLAS cannot distinguish between
row/column vectors.

Which means you are right, I can use A^T op() to get the same effect.

A^T * x^T = (x * A) ^T

x^T = x
(x * A)^T = x * A

so A^T * x = x * A

Thanks a lot!


On Tue, Oct 30, 2012 at 3:45 PM, Michael Lehn <address@hidden>wrote:

> Am 30.10.2012 um 14:35 schrieb Dimitris K.:
>
> For clarification, since I can't find anything in the docs, I want to ask.
>
>
> Say I want to compute the following:
>
>
> y = \alpha x op(A) + \beta y (where x,y are vectors and A is a matrix),
>
>
>
> So if op(A)=A your vectors are row-rectors.  In this case the operation is
> equivalent to
>
>  y = \alpha A^T  x + \beta y
>
> where x and y are column-vectors.  Other cases of op(A) can be treated
> analogously.
>
> As BLAS does not distinguish between col- and row-vectors you just can
> use gemv.
>
> If elements are complex valued there is a problem though.  Officially
> gemv in BLAS does not support an operation like
>
>  y = \alpha conj(A)  x + \beta y
>
> which would be required in your case if op(A)=A^H.  However, (I think)
> some BLAS implementation actually support it (TRANSA='R').
>
> Cheers,
>
> Michael
>



-- 
alkar