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# Re: Re[a + I b] = a
> how can I tell mathematica that the variable a and b is real, so that
> Re[a+ I b] results to a?
ComplexExpand assumes that a and b are real unless you tell it
otherwise.
In[2]:= ?ComplexExpand
ComplexExpand[expr] expands expr assuming that all variables are real.
ComplexExpand[expr, {x1, x2, ... }] expands expr assuming that
variables
matching any of the xi are complex.
In[3]:= ComplexExpand[ Re[a + I b] ]
Out[3]= a
> or: n a integer number and Sin[n x] results to 0?
>
> I am using mathematica 2.2
Algorithmically, this is a much harder (and largely unsolved) problem.
If your needs are not too elaborate, though, you may be able to program
it yourself, as in
In[4]:= Unprotect[Sin] ;
In[5]:= Sin[p_ Pi] := 0 /; IntegerQ[p]
In[6]:= Protect[Sin] ;
In[7]:= IntegerQ[n] ^= True ;
In[8]:= Sin[n Pi]
Out[8]= 0
Dave Withoff
Wolfram Research
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