MHB What is the cartesian product of two sets?

  • Thread starter Thread starter evinda
  • Start date Start date
  • Tags Tags
    Cartesian
evinda
Gold Member
MHB
Messages
3,741
Reaction score
0
Hey! (Nerd)

If $A,B$ are sets, the unique set $\{ <a,b>: a \in A \wedge b \in B \}$ is called cartesian product of $A,B$ and is symbolized as $A \times B$.

I want to find the cartesian product $\mathbb{Z} \times \{ 1, 2 \}$.
I thought, that it is equal to $\{ <x,1>,<x,2>: x \in \mathbb{Z}\}$.

According to my notes, it is equal to: $\{ <x,1>: x \in \mathbb{Z}\} \cup \{ <x,2>: x \in \mathbb{Z}\}$.

Are these the same? (Thinking)

Also, how can we show that $\varnothing \times A=A \times \varnothing=\varnothing$, right? (Worried)

It is: $\varnothing \times A=\{ <b,a>: b \in \varnothing, a \in A \}$?
Can we conclude from this, that $\varnothing \times A=\varnothing$ ? (Thinking)
 
Last edited:
Physics news on Phys.org
evinda said:
Hey! (Nerd)

If $A,B$ are sets, the unique set $\{ <a,b>: a \in A \wedge b \in B \}$ is called cartesian product of $A,B$ and is symbolized as $A \times B$.

I want to find the cartesian product $\mathbb{Z} \times \{ 1, 2 \}$.
I thought, that it is equal to $\{ <x,1>,<x,2>: x \in \mathbb{Z}\}$.

According to my notes, it is equal to: $\{ <x,1>: x \in \mathbb{Z}\} \cup \{ <x,2>: x \in \mathbb{Z}\}$.

Are these the same? (Thinking)

Hi! (Blush)

I have just looked it up in Set-builder notation on wiki.

It appears that formally the expression to the left of the ':' should represent one element and not two.
Since your notation is not ambiguous, I would still consider them the same.
But formally your first form is not defined. (Nerd)

Also, how can we show that $\varnothing \times A=A \times \varnothing=\varnothing$, right? (Worried)

It is: $\varnothing \times A=\{ <b,a>: b \in \varnothing, a \in A \}$?
Can we conclude from this, that $\varnothing \times A=\varnothing$ ? (Thinking)

Yes.
There is no element <b,a> that satisfies the conditions.
So the result is an empty set. (Nod)
 
I like Serena said:
It appears that formally the expression to the left of the ':' should represent one element and not two.
Since your notation is not ambiguous, I would still consider them the same.
But formally your first form is not defined. (Nerd)

So, is this: $\{ <a,b>,<c,d> : a,b,c,d \in \mathbb{Z}\}$ well-defined, but this: $\{ <a,b>,<a,d> : a,b,d \in \mathbb{Z}\}$ isn't? (Thinking)
Or have I understood it wrong? (Thinking)
And what if $x$ had a specific value? :confused:

I like Serena said:
Yes.
There is no element <b,a> that satisfies the conditions.
So the result is an empty set. (Nod)

So, can we say that since there is no element $b$, such that $b \in \varnothing$, there is no $<c,d>$, such that $<c,d> \in \{ <b,a>: b \in \varnothing, a \in A\}$, so $\forall <c,d>: <c,d> \notin \varnothing \times A$.
Therefore, $ \varnothing \times A=\varnothing$.

(Thinking)
 
evinda said:
So, is this: $\{ <a,b>,<c,d> : a,b,c,d \in \mathbb{Z}\}$ well-defined, but this: $\{ <a,b>,<a,d> : a,b,d \in \mathbb{Z}\}$ isn't? (Thinking)
Or have I understood it wrong? (Thinking)

Formally, neither is defined.
Informally, both are fine. (Smile)

And what if $x$ had a specific value? :confused:

Which x? :confused:
So, can we say that since there is no element $b$, such that $b \in \varnothing$, there is no $<c,d>$, such that $<c,d> \in \{ <b,a>: b \in \varnothing, a \in A\}$, so $\forall <c,d>: <c,d> \notin \varnothing \times A$.
Therefore, $ \varnothing \times A=\varnothing$.

(Thinking)

Yes, we can say that. (Mmm)
 
I like Serena said:
Formally, neither is defined.

Could you explain me further, why formally, neither is defined? (Thinking)

I like Serena said:
Which x? :confused:
From $\{ <x,1>, <x,2>: x \in \mathbb{Z} \}$.. :confused:
 
Hi all, I've been a roulette player for more than 10 years (although I took time off here and there) and it's only now that I'm trying to understand the physics of the game. Basically my strategy in roulette is to divide the wheel roughly into two halves (let's call them A and B). My theory is that in roulette there will invariably be variance. In other words, if A comes up 5 times in a row, B will be due to come up soon. However I have been proven wrong many times, and I have seen some...
Thread 'Detail of Diagonalization Lemma'
The following is more or less taken from page 6 of C. Smorynski's "Self-Reference and Modal Logic". (Springer, 1985) (I couldn't get raised brackets to indicate codification (Gödel numbering), so I use a box. The overline is assigning a name. The detail I would like clarification on is in the second step in the last line, where we have an m-overlined, and we substitute the expression for m. Are we saying that the name of a coded term is the same as the coded term? Thanks in advance.

Similar threads

Back
Top