x Turn on thread page Beta
 You are Here: Home >< Maths

# Logic Quantifiers watch

1. Having a look at some higher level calculus that i never came to through physics.
I am stuck conceptually at a first hurdle. If anyone can help me frame this in my mind it would be much appreciated.
I'm looking at the ordering of the universal and existential quantifiers.

I have this statement .
So for all x there exists a y that is greater than x. This is true.

However i don't get the statement .
There exists a y for all x that is greater than x. This is apparently false but i just can't seem to make it make sense for me.

Any help? Cheers
2. (Original post by stanante)
I have this statement .
So for all x there exists a y that is greater than x. This is true.
"for all" can be read as "for each", which sometimes makes the meaning clearer.

However i don't get the statement .
There exists a y for all x that is greater than x. This is apparently false but i just can't seem to make it make sense for me.

Any help? Cheers
That's the same logical statement.

I presume you meant

There exists a y such that for all x, x < y. Which means there is some number bigger than every other number. Which is clearly false - y is not bigger than y+1 for instance.
3. (Original post by ghostwalker)
That's the same logical statement.

I presume you meant

Yes i did. Sorry about that. I'll make an edit.

It's still not clicking. So there's a y that for every x can be bigger than x (x<y). What if x and y tend to infinity? Can there not be a y that is always larger?
4. (Original post by stanante)
Yes i did. Sorry about that. I'll make an edit.

It's still not clicking. So there's a y that for every x can be bigger than x (x<y). What if x and y tend to infinity? Can there not be a y that is always larger?
y doesn't tend anywhere. The proposition is stating that a y exists. So, what is it? It's not 1, 10^10, or any number. We can't put infinity there, as it's not a number.

What ever number we choose, we can always find a larger number by adding one to it.

Paraphrasing, it's saying there is a number bigger than all numbers.

Another counter example is whatever number we choose, it's not bigger than itself.
5. (Original post by ghostwalker)
y doesn't tend anywhere. The proposition is stating that a y exists. So, what is it? It's not 1, 10^10, or any number. We can't put infinity there, as it's not a number.

What ever number we choose, we can always find a larger number by adding one to it.

Paraphrasing, it's saying there is a number bigger than all numbers.

Another way of looking at it is whatever number we choose, it's not bigger than itself.
So basically, for all x, means 'all' numbers. So there can't be a number, y, larger than all numbers.
6. (Original post by stanante)
Yes i did. Sorry about that. I'll make an edit.

It's still not clicking. So there's a y that for every x can be bigger than x (x<y). What if x and y tend to infinity? Can there not be a y that is always larger?
Sometimes, it helps to think of like this:

You can think of it like this, someone is giving you an , it can be any number, and no matter what number they give you there is a that is bigger than it, and this works for any they give you.

This is clearly not true, because no matter what you set, if they gave you the number it doesn't work.
7. (Original post by Noble.)
Someone is giving you an , it can be any number, and no matter what number they give you there is a that is bigger than it, and this works for any they give you.

This is clearly not true, because no matter what you set, if they gave you the number it doesn't work.
How can we consider ? Isn't the statement that, a 'certain' is bigger than all . If we consider then is it not a different number being considered?
And if for a then is still bigger than an .

I understand what ghostwalker is saying that there can't be a larger than all numbers.

Although i get it once it's pointed out, it wasn't obvious to me.

I'm throwing it all out there. Sorry if it sounds ridiculous. Got to follow the routes coming up in my mind.
8. (Original post by stanante)
So basically, for all x, means 'all' numbers. So there can't be a number, y, larger than all numbers.
Yes, "all" means "all"
9. (Original post by stanante)
How can we consider ? Isn't the statement that, a 'certain' is bigger than all . If we consider then is it not a different number being considered?
And if for a then is still bigger than an .

I understand what ghostwalker is saying that there can't be a larger than all numbers.

Although i get it once it's pointed out, it wasn't obvious to me.

I'm throwing it all out there. Sorry if it sounds ridiculous. Got to follow the routes coming up in my mind.
Because can be any number, so they could say and if they gave you that which contradicts the statement.
10. Ok, this is cool now. Thanks to you both

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: April 5, 2013
Today on TSR

### Complete university guide 2019 rankings

Find out the top ten here

Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams