On Tue, Jul 10, 2012 at 7:05 AM, Damon McDougall
<damon.mcdoug...@gmail.com>wrote:
> On Tue, Jul 10, 2012 at 12:27:59PM +0200, Fabien Lafont wrote:
> > Hello everyone,
> >
> > I try to plot the digamma function of (1/2 + 1/x) but I'm not sure that
> I'm
> > plotting the good one.
> >
> > I've tried:
> >
> > special.polygamma(0, (1/2 + 1/x))
> >
> > and
> >
> > special.polygamma(1, (1/2 + 1/x))
>
> You want special.polygamma(0, (1/2 + 1/x)). See
>
> http://docs.scipy.org/doc/scipy/reference/generated/scipy.special.polygamma.html
>
> The number specifies which derivative of the digamma function you want.
> Surely you want the 0th derivative?
>
> > But It returns zero division error even when x is in ]0,1]
>
> I think it blows up at x = 0. What is the type of x in your usecase? Is
> it an array? If x contains the element 0, you will get a zero
> division error. You could try plotting the points explicitly:
>
> from numpy import linspace
> from pylab import *
>
> x = linspace(0.5, 2, num=100, endpoint=True)
> y = special.polygamma(0, (1/2 + 1/x))
> plot(x, y)
> show()
>
> You can compare output against this:
>
> http://www.wolframalpha.com/input/?i=digamma%281%2F2+%2B+1%2Fx%29+between+0.5+and+2
>
> Hope this helps.
>
>
Another problem might be the "1/2" part, which in python2.x would yield 0
unless one does "from __future__ import division".
Ben Root
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