       RE: Simplify

• To: mathgroup at smc.vnet.net
• Subject: [mg14793] RE: [mg14780] Simplify
• From: "Ersek, Ted R" <ErsekTR at navair.navy.mil>
• Date: Wed, 18 Nov 1998 01:28:55 -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:=
Clear[n,expr]

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

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

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

Cheers,
Ted Ersek

```

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