MHB What is the cartesian product of two sets?

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The Cartesian product of two sets A and B is defined as the set of ordered pairs {<a,b>: a ∈ A and b ∈ B}, denoted as A × B. The discussion revolves around finding the Cartesian product of the integers and the set {1, 2}, with the conclusion that both notations {<x,1>,<x,2>: x ∈ ℤ} and {<x,1>: x ∈ ℤ} ∪ {<x,2>: x ∈ ℤ} represent the same set. It is also clarified that the Cartesian product of an empty set with any set results in an empty set, as there are no elements to form pairs. The participants explore the formal definitions of set notation, concluding that while some expressions may be informally acceptable, they lack formal definition. The conversation emphasizes the importance of precise notation in set theory.
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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)
 
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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:
 

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