Services & Resources / Wolfram Forums / MathGroup Archive
-----

MathGroup Archive 2007

[Date Index] [Thread Index] [Author Index]

Search the Archive

Defining derivatives

  • To: mathgroup at smc.vnet.net
  • Subject: [mg84484] Defining derivatives
  • From: KF <fresl at grad.hr>
  • Date: Sat, 29 Dec 2007 02:55:22 -0500 (EST)

(I am sorry if this or similar question was already answered. I did 
archive search, but didn't find anything.)

I have a function:

   QuarticSpline[x_] :=
     Module[
       { r },
       r = Abs[x];
       If [r > 1.0,
           0.0,
           1 + r r (-6 + r (8 - 3 r))
       ]
     ]

It's first derivative should be (at least for my purposes :o) :

   QuarticSplineD1 [x_] :=
     Module[
       { r },
       r = Abs [x];
       If [r > 1.0,
           0.0,
           Sign[x] * r (-12 + (24 r - 12))
       ]
     ]

but D[QuarticSpline[x], x] returns

   If[Abs[x] > 1., 0, 0]

Now, Mathematica Tutorial, in chapter Defining Derivatives, says

"You can define the derivative in Mathematica of a function f of one 
argument simply by an assignment like f'[x_]=fp[x]."

But if I try to write

    QuarticSpline'[x_] = QuarticSplineD1[x]

I got

    Set::write: "Tag Function in (If[Abs[#1]>1.,0,0]&)[x_] is Protected."

Is it possible to define QuarticSpline' to "return" QuarticSplineD1 ?

Regards,

K. Fresl




  • Prev by Date: Re: Compiling Random Numbers from a distribution
  • Next by Date: Re: Creating an application
  • Previous by thread: Re: help with mathematica 5.2-Vector
  • Next by thread: Re: Defining derivatives