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MathGroup Archive 2000

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Re: Assumptions i Mathematica...

  • To: mathgroup at smc.vnet.net
  • Subject: [mg25700] Re: Assumptions i Mathematica...
  • From: Brian Higgins <bghiggins at ucdavis.edu>
  • Date: Thu, 19 Oct 2000 04:35:26 -0400 (EDT)
  • References: <8sjjc2$fu3@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Jes, One way to do this is to use the Condition function in the
definition of f(x): i.e.

f[x_/;x>0]:=x

The above function will only evaluate the RHS if  x>0.

In[80]:=f[.2]

Out[80]=0.2

In[81]:=f[-2]

Out[81]=f[-2]

 You can also add an additional definition for all other values

f[x/;<0]:=0

The down side with using this approach is that you cannot do
many symbolic operations on your function, e.g. differentiate etc.
An alternative method that gives you more flexibilitry is to use the
UnitStep function

In[89]:=f[x_] := x^3UnitStep[x]

In[91]:=D[f[x], x]

Out[91]=\!\(x\^3\ DiracDelta[x] + 3\ x\^2\ UnitStep[x]\)

Cheers,

Brian




In article <8sjjc2$fu3 at smc.vnet.net>,
  "Jes Hansen" <Whammer.antispam at post9.tele.dk> wrote:
> Hi, I'm quite new at this, and I havn't been able to figure out how
to tell
> Mathematica to assume that a variable, x, is positive. I have a
function,
> f(x), where I only want to consider positive values of x. I'm using
version
> 3.0.
>
> Regards
> Jes
>
>


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