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MathGroup Archive 2000

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Re: Piecewise functions definition and usage

  • To: mathgroup at smc.vnet.net
  • Subject: [mg24125] Re: Piecewise functions definition and usage
  • From: Brian Higgins <bghiggins at ucdavis.edu>
  • Date: Wed, 28 Jun 2000 02:11:45 -0400 (EDT)
  • References: <8j9ddk$50v@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Viorel, You may want to use  the UnitStep function to define your
discontinuous function:

In[20]:=f2[x_] = Sqrt[1 - ((x - 3)/10)^2]*(UnitStep[x + 7] -
    UnitStep[x - 13])

In[22]:=Integrate[f2[x], {x, -14, 14}]

Out[22]=5*Pi

Cheers,

Brian

In article <8j9ddk$50v at smc.vnet.net>,
  Viorel Ontalus <vio2 at mail.lehigh.edu> wrote:
> It seems I got into an area where Mathematica has some
problems, and I
> hope somebody can give me a hint on how to go around these
problems.
>
> 1.  I am trying to define a piecewise function and do some
computations
> with it. When I integrate mathematica does not behave. Here is
an simple
> example you can run and see what I am talking about
>
> Clear[f,x}
> f[x_] := Sqrt[ 1- ( (x-3)/10)^2  ] /;
> Abs[x-3]<10
> f[x_] := 0 /;
> Abs[x-3]>3 ;          (*this is a very simple piecewise function but
one
> must be sure the Sqrt is from a positive # )
>
> NIntegrate[f[x],{x,-1,17}]   (* Here Mathematica goes nuts !!!*)
>
> Of course it gives an answer but if your program is more
complex, then
> you never get an answer !!
> ( I tried to make the upper limit a variable  etc !!)
> Does anybody know how to avoid the error , or non convergence
messages I
> get !!
>
> 2. For fun I tried the only reference from the book on piecewise
> functions:
> If[Abs[x-3]<10, f[x_]:=Sqrt[1- ( (x-3)/10)^2  ] , f[x_]:=0 ]
>
> This definition does not work !!
>
>


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