Define the sum and the product of two cardinal numbers and show that these are well-defined operations.
That's what I have tried:
Let A,B sets with A \cap B=\varnothing, card(A)=m, card(B)=n.
We define the sum m+n of the cardinal numbers m,n as the cardinal number of the union of A and B, i.e.
m+n=card(A \cup B)
We will show that the sum of two cardinal numbers is well-defined.
It suffices to show that if A_1 \sim B_1, A_2 \sim B_2 with A_1 \cap A_2=\varnothing, B_1 \cap B_2=\varnothing then A_1 \cup A_2 \sim B_1 \cup B_2.
We know that there are bijective functions f: A_1 \to A_2, g:B_1 \to B_2.
We want to show that there is a bijective function h: A_1 \cup A_2 \to B_1 \cup B_2.
We set h(x)=f(x) if x \in A_1, h(x)=g(x) if x \in A_2.
We will show that h is 1-1.
Let x_1, x_2 \in A_1 \cup A_2 with h(x_1)=h(x_2).
If x_1, x_2 \in A_1 then h(x_1)=f(x_1), h(x_2)=f(x_2) and so f(x_1)=f(x_2) \Rightarrow x_1=x_2
If x_1, x_2 \in A_2 then h(x_1)=g(x_1), h(x_2)=g(x_2) and so g(x_1)=g(x_2) \Rightarrow x_1=x_2 since g is injective.
If x_1 \in A_1, x_2 \in A_2 then h(x_1)=h(x_2) \Rightarrow f(x_1)=f(x_2) that cannot be true because B_1 \cap B_2=\varnothing.
We will show that h is surjective, i.e. that \forall y \in B_1 \cup B_2, \exists x \in A_1 \cup B_2 such that f(x)=y.
If y \in B_1 then we know that there will be a x \in A_1 such that h(x)=y \Rightarrow f(x)=y since f is surjective.
If y \in B_2 then we know that there will be a x \in A_2 such that h(x)=y \Rightarrow g(x)=y since g is surjective.
We define the product m \cdot n of the cardinal numbers m,n as the cardinal number of the cartesian product of A and B, i.e.
m \cdot n=card(A \times B)
We will show that the product of two cardinal numbers is well-defined.
It suffices to show that if A_1 \sim B_1, A_2 \sim B_2 with A_1 \cap A_2=\varnothing, B_1 \cap B_2=\varnothing then A_1 \times A_2 \sim B_1 \times B_2.
We know that there are bijective functions f: A_1 \to A_2, g:B_1 \to B_2.
We want to show that there is a bijective function h: A_1 \times A_2 \to B_1 \times B_2.
We define h: A_1 \times A_2 \to B_1 \times B_2 such that \langle n,m \rangle \mapsto \langle f(n),g(m) \rangle
We will show that h is 1-1.
Let \langle m_1, n_1 \rangle, \langle m_2, n_2 \rangle \in A_1 \times A_2 with h(\langle m_1, n_1 \rangle)=h(\langle m_2, n_2 \rangle) \rightarrow \langle f(n_1),g(m_1) \rangle=\langle f(n_2),g(m_2) \rangle \rightarrow f(n_1))=f(n_2) \wedge g(m_1)=g(m_2) \overset{\text{f,g:} 1-1 }{\rightarrow} m_1=m_2 \wedge n_1=n_2 \rightarrow \langle m_1,n_1 \rangle=\langle m_2,n_2 \rangle
From the surjectivity of f,g we can conclude that h is surjective.
Could you tell me if it is right?