[SciPy-dev] Volunteer for Scipy Project

Anne Archibald peridot.faceted@gmail....
Mon Oct 5 20:29:52 CDT 2009

2009/10/5 Charles R Harris <charlesr.harris@gmail.com>:
> On Mon, Oct 5, 2009 at 3:20 PM, Anne Archibald <peridot.faceted@gmail.com>
> wrote:
>> 2009/10/5 David Goldsmith <d.l.goldsmith@gmail.com>:
>> > Just curious, Anne: have you anything in particular in mind (i.e., are
>> > there
>> > some small - or gaping - holes in scipy (IYO, of course) which you know
>> > could be filled by a careful implementation of something(s) extant in
>> > the
>> > literature)?
>> Well, not exactly - the examples I had in mind were minor and/or in
>> the past. I ran into problems with scipy's hyp2f1, for example, so I
>> went and looked up the best algorithm I could find for it (and I think
>> I contributed that code). I wanted the Kuiper test as an alternative
>> to the Kolmogorov-Smirnov test (it's invariant under cyclic
>> permutations, and is sensitive to different features of the
>> distribution) so I looked up the test and the special function needed
>> to interpret its results. (I haven't contributed this to scipy yet,
>> mostly because I chose an interface that's not particularly compatible
>> with that for scipy's K-S test.) And on a larger scale, that's what
>> scipy.spatial's kdtree implementation is.
>> For examples where I think a bit of lit review plus implementation
>> work might help, I'd say that the orthogonal polynomials could use
>> some work - the generic implementation in scipy.special falls apart
>> rapidly as you go to higher orders. I always implement my own
>> Chebyshev polynomials using the cos(n*arccos(x)) expression, for
>> example, and special implementations for the others might be very
>> useful.
> At what order does the scipy implementation of the Chebyshev polynomials
> fall apart? I looked briefly at that package a long time ago, but never used
> it. I ask so I can check the chebyshev module that is going into numpy.

By n=30 they are off by as much as 0.0018 on [-1,1]; n=31 they are off
by 0.1, and by n=35 they are off by four - not great for values that
should be in the interval [-1,1]. This may seem like an outrageously
high degree for a polynomial, but there's no reason they need be this
bad, and one could quite reasonably want to use an order this high,
say for function approximation.

I think this inaccuracy is probably inevitable in a scheme that
computes values using a recurrence relation, and something like it
probably occurs for all the orthogonal polynomials that don't have
special-purpose evaluators.


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