Re: How does a rule get applied?
- To: mathgroup@smc.vnet.net
- Subject: [mg12485] Re: How does a rule get applied?
- From: "Allan Hayes" <hay@haystack.demon.cc.uk>
- Date: Tue, 19 May 1998 13:31:48 -0400
- References: <6jf2mg$3vl@smc.vnet.net>
Yacine Ait-Sahalia gives the following example:
x^a + x^(1+a) /. x^(n_) + x^(n_+1) ->0
x^3 + x^(1+3) /. x^(n_) + x^(n_+1) ->0
x^3 /. x^(n_) ->0
Out[388]=
0
Out[389]=
3 4
x + x
Out[390]=
0
And asks why the second output is not 0.
Yacine:
x^3 + x^(1+3) evaluates to x^3 + x^4 before replacement is tried, so
there is no match.
To get round this we can temporarily hold until replacement is done:
Unevaluated[x^3 + x^(1+3)] /. x^(n_) + x^(n_+1) ->0
0
Allan Hayes
Training and Consulting
Leicester UK
http://www.haystack.demon.co.uk
hay@haystack.demon.co.uk
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