Re: [Help-gsl] Necessity of providing f, df, _and_ fdf?

[Brian Gough]

At Mon, 19 Jan 2009 18:20:56 -0500,
Liam Healy wrote:
> Is possible to change GSL to allow f & df _or_ fdf instead of "and",
> say providing a null pointer for the unneeded function.  If that's
> already the case, can the documentation be made clear?

Hello,

To keep things simple using a NULL pointer is not supported -- the
recommended way is to make a small function fdf which calls f and df.

> In a related puzzle, I find this statement about fdf hard to understand:
> "This function provides an optimization of the separate functions for
> f(x) and g(x)—it is always faster to compute the function and its
> derivative at the same time. "
> The user is providing the function, so whether it's an optimization or
> not depends on how it's written.  It seems like an overstatement that
> it's "always" faster to compute the function and its derivative at the
> same time; I'm willing to believe it's (essentially) at least as fast
> perhaps.

I was thinking that for analytic functions, at least, the function and
the derivative always have some terms in common so it is faster to
compute them together.

-- 
Brian Gough