[R] roots of a function

[Martin Maechler]

>>>>> "Alejandro" == Alejandro Veen <veen at stat.ucla.edu>
>>>>>     on Mon, 14 Nov 2005 10:23:02 -0800 writes:

    Alejandro> For finding the root of the following function I
    Alejandro> have been using 'uniroot': f(p) = log(p-1) -
    Alejandro> log(p) + 1/(p-1) - log(A) - B = 0

    Alejandro> where 'p' is a scalar.  However, I will have to
    Alejandro> find this root repeatedly, so I would like to
    Alejandro> suggest a starting value, which is not possible
    Alejandro> with 'uniroot'.  'nlm' allows the use of starting
    Alejandro> values, so I have been thinking of applying 'nlm'
    Alejandro> to abs(f(p)).  Is that the way to go or is there
    Alejandro> a better way I don't know about?

No, using minimization instead of root finding is typically not
as efficient (particularly in one dimension).

But in some ways you *have* been using  starting values for
uniroot() contrary to what you said:

uniroot even needs an starting *interval* in which to search.
If you know a lot about your function and its zero, just make
that initial interval appropriately small.

    Alejandro> Thanks for your help,

you're welcome!
Martin