Dear Andre: Brevity is the usual reason for introducing new terminology. And, ultimately there is a need, as mathematics would grind to a halt if one had to write out a phrase giving the content of each concept whenever one wanted to talk about it. One may object to the jocular use of a word with a moral denotation as the name for a mathematical concept, but four characters beats 28 characters (or 31 if one counts spaces). It certainly seems desirable to have a brief name for either "invariant under equivalence" or "not invariant under equivalence". Best Thoughts, David Y. On 18 Sep 2010, at 08:50, Joyal, André wrote:
Dear John,
A property is "evil" in your sense if it is not invariant under equivalences. Invariance under equivalence is a well established mathematical notion. I prefer to say that something is not invariant under equivalence than to say that it is "evil". There is no need to introduce a new terminology.
Best, André
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