Tag Archives: For All

A quick note on quantifiers: For almost all and there exist infinitely many

It's been a while since my last post and now it's time for a mathematical post:

I'm currently preparing for a math exam (calculus) and I'm thinking it would be nice if there was a way to avoid much of the "for every \epsilon > 0 there is an N \in \mathbb{N}, so that for all n \ge N some property ... holds" stuff you find in textbooks. Some textbooks actually shorten it to "for every \epsilon > 0 some property ... holds for almost all n".
However, I haven't found a quantifier to express this anywhere yet, so I'm proposing to introduce a new one:

\tilde{\forall} x \in M: P(x) should mean "for almost all x \in M P(x) holds", which suggests that there are only finitely many elements for which it does not hold.
One can formalize this as:
\tilde{\forall} x \in M: P(x) \equiv \exists n \in \mathbb{N}: \left | \left \{ x \in M \mid \neg P(x) \right \} \right | \le n

Of course, a new existential quantifier is required then, too (for negation):
\tilde{\exists} x \in M: P(x) stands for "there exist infinitely many x \in M, for which P(x) holds".
And this can be formalized as:
\tilde{\exists} x \in M: P(x) \equiv \forall n \in \mathbb{N}: \left | \left \{ x \in M \mid P(x) \right \} \right | > n

It's easy to see that \neg \left ( \tilde{\forall} x \in M: P(x) \right ) \equiv \tilde{\exists} x \in M: \neg P(x)

Thus one can use the two quantifiers just as one would use \forall and \exists usually. Note however than \forall and \tilde{\forall} don't interchange and neither do \exists and \tilde{\exists}.

One last note: it might be worth using a different notation, for example: \exists ^ \infty might be easier to understand than  \tilde{\exists} , and \forall ^ \approx might be better than \tilde{\forall}.

I'll try to formalize these quantifiers some more when I find some spare time.

Stay tuned for coding related updates soon :-)
Cheers,
Andreas