Re: algebraic substitution rules

• To: mathgroup at smc.vnet.net
• Subject: [mg30736] Re: algebraic substitution rules
• From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
• Date: Sun, 9 Sep 2001 03:26:25 -0400 (EDT)
• Organization: Universitaet Leipzig
• References: <9ncfq6\$pt5\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Hi,

the onlyne help say:

ReplaceAll looks at each part of expr, tries all the rules on it,
and then goes on to the next part of expr.
The first rule that applies to a particular part is used;
********************************************************
no further rules are tried on that part,
***************************************
or on any of its subparts

since a_ match to the full expression the rule a_->Expand[a]
is used and no future rules are tried on that part.

You mean

1 + x^2 + x^3 + x^4 /.
{x^2 -> 1 + y, x^3 -> y(1 + y), x^4 -> (1 + y)^2} /.
a_ :> Expand[a]

Regards
Jens

Cattiaux Isabelle wrote:
>
>
> Hi,
>
>   Could someone tell me why the first substitution rule
>  works and the second doesn't
>
> In[1]:==
> 1+x^2+x^3+x^4 /. {x^2->1+x ,x^3->x(1+x) ,x^4->(1+x)^2}
>
> Out[1]==
> 2 + x + x(1 + x)+ (1 + x)^2
>
> In[78]:==
> 1+x^2+x^3+x^4 /. {x^2->1+x ,x^3->x(1+x) ,x^4->(1+x)^2,a_->Expand[a]}
>
> Out[78]==
> 1 + x^2 + x^3 + x^4
>
> --
> Isabelle Cattiaux-Huillard
> Universite de Valenciennes

```

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