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]

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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