I wanted to receive some help regarding some homework questions; I appreciate any sort of feedback possible.
I am certain that any Dedekind Cut $A$ has the following properties:
1) $A$ is not the empty set and is NOT the set $\mathbb Q$. 2) For any $a \in A$ and $b$ $\in \mathbb Q$ \ A, $a<b$. 3) $A$ has no maximal element.
Working on defining addition and multiplication for Dedekind Cuts (as the set of all dedekind cuts with the order $\subseteq$ is the set $\mathbb R$.
The additive identity is defined as: $0_\mathbb R$ $:=$ {$y$ $\in \mathbb Q$ such that $y < 0$}.
Question 1: I need to prove that for any Dedekind Cuts $X$ and $Y$, that $X$ + $0_\mathbb R$ $= X$.
This requires me to show that they are both subsets of each other (to be true). By the definition of addition of Dedekind cuts, we have:
$X$ + $0_\mathbb R$ $:=$ {$z$: $z \in \mathbb Q$ such that $z=x+y$, where $x \in X$ and $y \in$ $0_\mathbb R$}.
To show that $X$ + $0_\mathbb R$ $\subseteq X$, we know that for any $z$ in $X$ + $0_\mathbb R$ that $\exists$ $x<a, y<0$ such that $z=x+y$. This implies that $z<a$.
To show that $X \subseteq$ $X$ + $0_\mathbb R$, there exist elements in $0_\mathbb R$ such that they are not contained in $X$. Is this valid?
Question 2: With the multiplication of 2 Dedekind Cuts $X$ and $Y$, I need to show that $XY$ $:=$ { $z$ $\in \mathbb Q:$ there exist $a \in$ $X ∩ \mathbb Q+$, $b \in Y ∩ \mathbb Q+$ such that $z ≤ ab$} is also a Dedekind Cut.
Thus there are three things to show:
a)$\mathbb Q$ \ $XY$ is non empty.
Proof by Contradiction. Assume otherwise, that $\mathbb Q$ \ $XY$ is empty. Then $\mathbb Q \subseteq XY$. Then $∃ z ∈ XY$ such that $z$ is NOT in $\mathbb Q$. Let $z ≤ xy$, $∀ x ∈ X, y ∈ Y$.
Then any $x$ can be represented as $a/b$ and the same with $y$ as $c/d$, where $a,b,c,d ∈ \mathbb Z$, $b,d ≠ 0$.
Then $xy = ab/cd \in \mathbb Q$. But since $z ≤ ac/bd \in \mathbb Q$, then $z ∈ \mathbb Q$ (which is a contradiction).
Thus no such $z$ can exist and so $\mathbb Q$ \ $XY$ is non-empty.
b) Need to show that $∀ z∈ XY$, $∃ z′∈ \mathbb Q$ \ $XY$ such that $z < z′$.
I am having trouble with this part.
c) Need to show that $XY$ has no maximal element.
I am also having trouble with this part.
3) After defining the multiplication of 2 Dedekind Cuts, I am asked to show that the additive inverse, $1_\mathbb R$ exists; that is,
where $1_\mathbb R$ $:=$ {$y \in \mathbb Q$, $y < 0$}.
Prove that $1X=X$.
This requires us to show that $1X ⊆ X$ and vice versa, I assume.