Edsger Dijkstra wrote a famous letter in the Communications of the ACM that appeared under the heading "Go To Statement Considered Harmful".
I'd like to make a case against the use of "any" in mathematical discourse.
The problem with "any" is that it can mean both "for all" and "there exists", and it's not always clear what is meant. "It's true for any x" probably means "for all x". But "The theorem is true for S if any element of S is a square" probably means "it's true if S contains at least one square".
I was just attending a meeting at Dagstuhl in Germany where one speaker said something like "If L is regular, then u is equivalent to v if and only if for any state q of the minimal DFA for L we have δ(q,u) = δ(q,v)". Now if you know the theorem, the meaning is clear. But if you don't, you might be left wondering, does he mean "u is equivalent to v if there exists some state q such that δ(q,u) = δ(q,v)" or "u is equivalent to v if for all states q we have δ(q,u) = δ(q,v)"?
Because of this ambiguity, I think we should avoid the use of "any" in mathematical discourse. We can replace it by "all x" or by "some x", according to what we mean.
Who's with me?
Showing posts with label proofs. Show all posts
Showing posts with label proofs. Show all posts
Friday, February 25, 2011
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