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Re: Domain of a function

  • To: mathgroup at smc.vnet.net
  • Subject: [mg122857] Re: Domain of a function
  • From: Peter Pein <petsie at dordos.net>
  • Date: Sat, 12 Nov 2011 07:36:22 -0500 (EST)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <j9b6lb$4sv$1@smc.vnet.net>

Am 08.11.2011 13:17, schrieb Tutor:
> Hallo!
>
> How can I find the domain of a function?
>
> something like this: ForAll[Element[f,Reals]] and it should return
> something like this: x element ]-2,oo[
>
> I have NO idea, how this can be done for a function f...
>

With a little luck, Reduce can do this:


In[20]:= Reduce[Element[x | Sqrt[2 + x], Reals], x]
Out[20]= x >= -2
In[21]:= Reduce[Element[x | Sqrt[1/2 + Sin[x]], Reals, x, Reals]
Out[21]= (Element[C[1], Integers] && (x == -Pi/6 + 2 Pi C[1] || x== 7 
Pi/6 + 2 Pi C[1])) ||
  (Element[C[1], Integers] && 1/6 (-Pi + 12 Pi C[1]) < x < 1/6 (7 Pi + 
12  Pi C[1]))



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