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MathGroup Archive 1998

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RE: Simplify

  • To: mathgroup at smc.vnet.net
  • Subject: [mg14794] RE: [mg14780] Simplify
  • From: "Ersek, Ted R" <ErsekTR at navair.navy.mil>
  • Date: Wed, 18 Nov 1998 01:28:56 -0500
  • Sender: owner-wri-mathgroup at wolfram.com

Jurgen Tischer  wrote:
>
>Could someone tell me please how to simplify the following formula?
>
>(-7253*2^(1 + 2*n)*5^n - 523*4^n*5^(1 + n) + 17121*20^n + 
>    130321*20^n*n^4)/130321
>
>To my surprise when I changed by hand (with two rules) the powers of 2,
>5 and so on to powers of 20, Mathematica changed them back to this
>form.
>
>
It seems you have to use HoldForm in the rules as I do below.

In[1]:=
Clear[n,expr]

In[2]:=
expr=(-7253*2^(1 + 2*n)*5^n - 523*4^n*5^(1 + n) + 17121*20^n + 
    130321*20^n*n^4)/130321;


In[3]:=
expr/.{
c_*4^n*5^(1+n):>5*c*HoldForm[20^n],
c_*2^(1+2 n)5^n:>2*c*HoldForm[20^n], 20^n:>HoldForm[20^n]}
Out[3]=
n^4 20^n


You need to use HoldForm because the kernel wants to be clever and undo
your rules.  Try evaluating the cells below.  I would classify this as
a bug.

In[4]:=
4 20^n
Out[4]=
4^(1 + n)*5^n   (* Arrrgh!  *)

In[5]:=
5 20^n
Out[5]=
4^n*5^(1 + n)   (* Arrrgh!  *)

	Cheers,
	Ted Ersek


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