Mailing-List: contact dev-help@commons.apache.org; run by ezmlm
Precedence: bulk
Reply-To: "Commons Developers List" <dev@commons.apache.org>
Received-SPF: pass (athena.apache.org: domain of ajo.fod@gmail.com designates
 209.85.128.50 as permitted sender)
MIME-Version: 1.0
In-Reply-To: <521A6718.9010100@spaceroots.org>
References: 
 <CAOOSspjCPhzSydpsNbqKn_zJzYR=N4krEHnGK++akUO6UYL-vA@mail.gmail.com>
	<da682c76d2b6e0645ed9a154f2a0f478@scarlet.be>
	<CAOOSspgnwZMufhmRPyjPZeCW8YJMQOw9dRjhKHt8HQP28yCrXA@mail.gmail.com>
	<5217903D.7020407@free.fr>
	<CAOOSsph7gh8Pd2J8Uom-k7gq3F3+xbhPKgB5pxCDyWTOUgJdVA@mail.gmail.com>
	<52187BC5.9090301@free.fr>
	<521A6718.9010100@spaceroots.org>
Date: Sun, 25 Aug 2013 16:19:16 -0700
Message-ID: 
 <CAOOSsphycr9D9ZWvz=F=YWG3H7obu=xtZpuFqgnXXQrqMQSY_w@mail.gmail.com>
Subject: Re: [MATH] What is the derivative of 0^x
From: Ajo Fod <ajo.fod@gmail.com>
To: Commons Developers List <dev@commons.apache.org>
Content-Type: multipart/alternative; boundary=001a11c2e3925e507304e4cddee9

--001a11c2e3925e507304e4cddee9
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable

Are you saying patched the code? Can you provide the link?

-Ajo


On Sun, Aug 25, 2013 at 1:20 PM, Luc Maisonobe <luc@spaceroots.org> wrote:

> Le 24/08/2013 11:24, Luc Maisonobe a =E9crit :
> > Le 23/08/2013 19:20, Ajo Fod a =E9crit :
> >> Hello,
> >
> > Hi Ajo,
> >
> >>
> >> This shows one way of interpreting the derivative for strictly +ve
> numbers.
> >>
> >>     public static void main(final String[] args) {
> >>         final double x =3D 1d;
> >>         DerivativeStructure dsA =3D new DerivativeStructure(1, 1, 0, x=
);
> >>         System.out.println("Derivative of |a|^x wrt x");
> >>         for (int p =3D 10; p < 21; p++) {
> >>             double a;
> >>             if (p < 20) {
> >>                 a =3D 1d / Math.pow(2d, p);
> >>             } else {
> >>                 a =3D 0d;
> >>             }
> >>             final DerivativeStructure a_ds =3D new DerivativeStructure=
(1,
> 1,
> >> a);
> >>             final DerivativeStructure out =3D a_ds.pow(dsA);
> >>             final double calc =3D (Math.pow(a, x + EPS) - Math.pow(a, =
x))
> /
> >> EPS;
> >>             System.out.format("Derivative@%f=3D%f  %f\n", a, calc,
> >> out.getPartialDerivative(new int[]{1}));
> >>         }
> >>     }
> >>
> >> At this point I"m explicitly substituting the rule that
> derivative(|a|^x) =3D
> >> 0 for |a|=3D0.
> >
> > Yes, but this fails for x =3D 0, as the limit of the finite difference =
is
> > -infinity and not 0.
> >
> > You can build your own function which explicitly assumes a is constant
> > and takes care of special values as follows:
> >
> >  public static DerivativeStructure aToX(final double a,
> >                                         final DerivativeStructure x) {
> >      final double lnA =3D (a =3D=3D 0 && x.getValue() =3D=3D 0) ?
> >                   Double.NEGATIVE_INFINITY :
> >                   FastMath.log(a);
> >      final double[] function =3D new double[1 + x.getOrder()];
> >      function[0] =3D FastMath.pow(a, x.getValue());
> >      for (int i =3D 1; i < function.length; ++i) {
> >          function[i] =3D lnA * function[i - 1];
> >      }
> >      return x.compose(function);
> >  }
> >
> > This will work and provides derivatives to any order for almost any
> > values of a and x, including a=3D0, x=3D1 as in your exemple, but also
> > slightly better for a=3D0, x=3D0. However, it still has an important
> > drawback: it won't compute the n-th order derivative correctly for a=3D=
0,
> > x=3D0 and n > 1. It will provide NaN for these higher order derivatives
> > instead of +/-infinity according to parity of n.
>
> I have added a similar function to the DerivativeStructure class (with
> some errors above corrected). The main interesting property of this
> function is that it is more accurate that converting a to a
> DerivativeStructure and using the general x^y function. It does its best
> to handle the special case, but as written above, this does NOT work for
> general combination (i.e. more than one variable or more than one
> order). As soon as there is a combination, the derivative will involve
> something like df/dx * dg/dy and as infinities and zeros are everywheren
> NaN appears immediately for these partial derivatives. This cannot be
> avoided.
>
> If you stay away from the singularity, the function behaves correctly.
>
> best regards,
> Luc
>
> >
> > This is a known problem that we already encountered when dealing with
> > rootN. Here is an extract of a comment in the test case
> > testRootNSingularity, where similar NaN appears instead of +/- infinity=
.
> > The dsZero instance in the comment is simple the x parameter of the
> > function, as a derivativeStructure with value 0.0 and depending on
> > itself (dsZero =3D new DerivativeStructure(1, maxOrder, 0, 0.0)):
> >
> >
> > // the following checks shows a LIMITATION of the current implementatio=
n
> > // we have no way to tell dsZero is a pure linear variable x =3D 0
> > // we only say: "dsZero is a structure with value =3D 0.0,
> > // first derivative =3D 1.0, second and higher derivatives =3D 0.0".
> > // Function composition rule for second derivatives is:
> > // d2[f(g(x))]/dx2 =3D f''(g(x)) * [g'(x)]^2 + f'(g(x)) * g''(x)
> > // when function f is the nth root and x =3D 0 we have:
> > // f(0) =3D 0, f'(0) =3D +infinity, f''(0) =3D -infinity (and higher
> > // derivatives keep switching between +infinity and -infinity)
> > // so given that in our case dsZero represents g, we have g(x) =3D 0,
> > // g'(x) =3D 1 and g''(x) =3D 0
> > // applying the composition rules gives:
> > // d2[f(g(x))]/dx2 =3D f''(g(x)) * [g'(x)]^2 + f'(g(x)) * g''(x)
> > //                 =3D -infinity * 1^2 + +infinity * 0
> > //                 =3D -infinity + NaN
> > //                 =3D NaN
> > // if we knew dsZero is really the x variable and not the identity
> > // function applied to x, we would not have computed f'(g(x)) * g''(x)
> > // and we would have found that the result was -infinity and not NaN
> >
> > Hope this helps
> > Luc
> >
> >>
> >> Thanks,
> >> Ajo.
> >>
> >>
> >>
> >> On Fri, Aug 23, 2013 at 9:39 AM, Luc Maisonobe <Luc.Maisonobe@free.fr
> >wrote:
> >>
> >>> Hi Ajo,
> >>>
> >>> Le 23/08/2013 17:48, Ajo Fod a =E9crit :
> >>>> Try this and I'm happy to explain if necessary:
> >>>>
> >>>> public class Derivative {
> >>>>
> >>>>     public static void main(final String[] args) {
> >>>>         DerivativeStructure dsA =3D new DerivativeStructure(1, 1, 0,
> 1d);
> >>>>         System.out.println("Derivative of constant^x wrt x");
> >>>>         for (int a =3D -3; a < 3; a++) {
> >>>
> >>> We have chosen the classical definition which implies c^x is not
> defined
> >>> for real r and negative c.
> >>>
> >>> Our implementation is based on the decomposition c^r =3D exp(r * ln(c=
)),
> >>> so the NaN comes from the logarithm when c <=3D 0.
> >>>
> >>> Noe also that as explained in the documentation here:
> >>> <
> >>>
> http://commons.apache.org/proper/commons-math/userguide/analysis.html#a4.=
7_Differentiation
> >>>> ,
> >>> there are no concepts of "constants" and "variables" in this framewor=
k,
> >>> so we cannot draw a line between c^r as seen as a univariate function
> of
> >>> r, or as a univariate function of c, or as a bivariate function of c
> and
> >>> r, or even as a pentavariate function of p1, p2, p3, p4, p5 with both=
 c
> >>> and r being computed elsewhere from p1...p5. So we don't make special
> >>> cases for the case c =3D 0 for example.
> >>>
> >>> Does this explanation make sense to you?
> >>>
> >>> best regards,
> >>> Luc
> >>>
> >>>
> >>>>             final DerivativeStructure a_ds =3D new
> DerivativeStructure(1,
> >>> 1,
> >>>> a);
> >>>>             final DerivativeStructure out =3D a_ds.pow(dsA);
> >>>>             System.out.format("Derivative@%d=3D%f\n", a,
> >>>> out.getPartialDerivative(new int[]{1}));
> >>>>         }
> >>>>     }
> >>>> }
> >>>>
> >>>>
> >>>>
> >>>> On Fri, Aug 23, 2013 at 7:59 AM, Gilles <gilles@harfang.homelinux.or=
g
> >>>> wrote:
> >>>>
> >>>>> On Fri, 23 Aug 2013 07:17:35 -0700, Ajo Fod wrote:
> >>>>>
> >>>>>> Seems like the DerivativeCompiler returns NaN.
> >>>>>>
> >>>>>> IMHO it should return 0.
> >>>>>>
> >>>>>
> >>>>> What should be 0?  And Why?
> >>>>>
> >>>>>
> >>>>>
> >>>>>> Is this worthy of an issue?
> >>>>>>
> >>>>>
> >>>>> As is, no.
> >>>>>
> >>>>> Gilles
> >>>>>
> >>>>>
> >>>>>> Thanks,
> >>>>>> -Ajo
> >>>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>
> ------------------------------**------------------------------**---------
> >>>>> To unsubscribe, e-mail: dev-unsubscribe@commons.**apache.org<
> >>> dev-unsubscribe@commons.apache.org>
> >>>>> For additional commands, e-mail: dev-help@commons.apache.org
> >>>>>
> >>>>>
> >>>>
> >>>
> >>>
> >>> ---------------------------------------------------------------------
> >>> To unsubscribe, e-mail: dev-unsubscribe@commons.apache.org
> >>> For additional commands, e-mail: dev-help@commons.apache.org
> >>>
> >>>
> >>
> >
> >
> > ---------------------------------------------------------------------
> > To unsubscribe, e-mail: dev-unsubscribe@commons.apache.org
> > For additional commands, e-mail: dev-help@commons.apache.org
> >
> >
>
>
> ---------------------------------------------------------------------
> To unsubscribe, e-mail: dev-unsubscribe@commons.apache.org
> For additional commands, e-mail: dev-help@commons.apache.org
>
>

--001a11c2e3925e507304e4cddee9--