• Oct 4th 2012, 12:51 AM
mastermind2007
Hello guys,

since there is no section "Topology" or "Logic", I think "Advanced Algebra" is yet the closest.
I've come upon the term "filter" in Mathematics. Principally, I have no problem understanding it - however there is one point in the
definition that I don't understand. It is as follows:

Be $\displaystyle (P, \leq)$ a partially ordered set and F a subset. Then F is a filter iff

1. ...

2. $\displaystyle \forall x,y \in F \ \exists \ z \in F : z \leq x \ \text{and} \ z \leq y$

3. ...

Now the question arises with 2.: Why do I need two elements x,y to be greater or equal to z? Why is one not enough, and why do I not need three or something? What exactly does that mean?

• Oct 4th 2012, 01:16 AM
chiro
Hey mastermind2007.

I'm wondering if there are two different kinds of orders here of if the partial order used refers to the same order.

Also do you know the partial order properties of x with respect to y?

The z <= x and z <=y implies that x <= y if we are talking about the same partial order. This means that there is a point somewhere where everything is "less than" some particular value of y which means that if a <= y then everything gets filtered on this condition.

I get the feeling this is needed because of the partial ordering axiom, but I'm not completely sure since this rule is part of the ordering axioms themselves.

When I read the definition, it seems that all this thing is doing is classifying one subset to be <=, it's complement to be not part of the filter and then keeping the subset that satisfies the partial order with respect to some particular reference point, where the ordering can be whatever the hell it wants to be as long as it divides the set into two groups (filtered and non-filtered) and carries some additional ordering within the filtered subset characterized by the partial order itself.
• Oct 4th 2012, 01:23 AM
emakarov
Questions about logic should be posted in the Discrete Mathematics forum (its subtitle is "Discrete mathematics, logic, set theory").

Quote:

Originally Posted by mastermind2007
2. $\displaystyle \forall x,y \in F \ \exists \ z \in F : z \leq x \ \text{and} \ z \leq y$

Now the question arises with 2.: Why do I need two elements x,y to be greater or equal to z? Why is one not enough, and why do I not need three or something?

The property $\displaystyle \forall x \in F \ \exists z \in F : z \leq x$ is not a restriction because one can take z = x. On the other hand, it is easy to see by induction using the transitivity of ≤ that if a lower bound of any two elements of F is in F, then a lower bound of any finite number of elements of F is again in F.

Quote:

Originally Posted by chiro
The z <= x and z <=y implies that x <= y if we are talking about the same partial order.

Why?
• Oct 4th 2012, 02:15 AM
chiro