Re: how to define a constant like Pi in Mathematica
- To: mathgroup at smc.vnet.net
- Subject: [mg63950] Re: how to define a constant like Pi in Mathematica
- From: Bill Rowe <readnewsciv at earthlink.net>
- Date: Tue, 24 Jan 2006 01:30:39 -0500 (EST)
- Sender: owner-wri-mathgroup at wolfram.com
On 1/23/06 at 4:11 AM, doesnotexist at dread13.news.tele.dk (John
Smith) wrote:
>I would like to define a constant in mathematica for example
>c=1.2345 in such a way that it behaves exactly like Pi or E. That
>is when I type c then I get the output c and not 1.2345. If I do
>integration[Exp[-c*x^2],{x,-oo,oo}] then I should get sqrt[Pi/c]
>and not If[c<0...]
You should be able to assign a value to c and have c displayed rather than the value using Hold or one of the variants of Hold. But this really won't achieve what you want since you still need to have Integrate determine c is a real > 0.
Rather than trying to get c to behave this way I believe it is far simpler and easier to use Assuming, i.e.,
In[14]:=
Assuming[c > 0, Integrate[Exp[(-c)*x^2], {x, -Infinity, Infinity}]]
Out[14]=
Sqrt[Pi]/Sqrt[c]
And if I needed to convert this to a number, then
Assuming[c > 0, Integrate[Exp[(-c)*x^2], {x, -Infinity, Infinity}]]/.c->1.2345
suffices. Or probably even better would be to use NIntegrate instead of Integrate where a number is desired rather than a symbolic solution.
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