# Greater than or equal to

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## Definition

Greater than or equal to is a relation (specifically a partial ordering) on [ilmath]\mathbb{R} [/ilmath] (and thus [ilmath]\mathbb{Q} [/ilmath], [ilmath]\mathbb{Z} [/ilmath] and [ilmath]\mathbb{N} [/ilmath]).

TODO: Link with ordered integral domain (as that is where the ordering is induced) THE STRUCTURE ON [ilmath]\mathbb{R} [/ilmath] IS VERY IMPORTANT. For example the epsilon form below requires addition, subtraction, so forth

## Alternative forms

Epsilon form: [ilmath]x\ge y\iff\forall\epsilon>0[x+\epsilon>y][/ilmath]

[ilmath]x\ge y\implies\forall\epsilon>0[x+\epsilon>y][/ilmath]

• Let [ilmath]\epsilon > 0[/ilmath] be given
• As [ilmath]\epsilon>0[/ilmath] we see [ilmath]x+\epsilon>0+x=x[/ilmath]
• But by hypothesis [ilmath]x\ge y[/ilmath]
• So [ilmath]x+\epsilon>x\ge y[/ilmath]
• Thus [ilmath]x+\epsilon>y[/ilmath]
• This completes this part of the proof.

[ilmath]\forall\epsilon>0[x+\epsilon>y]\implies x \ge y[/ilmath] (this will be a proof by contrapositive)

• We will show: [ilmath]x<y\implies\exists\epsilon>0[x+\epsilon < y][/ilmath] Warning:I wrongly negated [ilmath]>[/ilmath], it should be [ilmath]\le[/ilmath] not [ilmath]<[/ilmath] - in light of this I might be able to get away with [ilmath]\epsilon=y-x[/ilmath]
• As [ilmath]x<y[/ilmath] we know [ilmath]0<y-x[/ilmath].
• Choose [ilmath]\epsilon:=\frac{y-x}{2}[/ilmath] (which we may do for both [ilmath]\mathbb{R} [/ilmath] and [ilmath]\mathbb{Q} [/ilmath])
• Now [ilmath]x+\epsilon=\frac{2x}{2}+\frac{y-x}{2}=\frac{x+y}{2}[/ilmath]
• But by hypothesis [ilmath]x<y[/ilmath] so [ilmath]x+y<y+y=2y[/ilmath], so:
• [ilmath]x+\epsilon=\frac{x+y}{2}<\frac{2y}{2}=y[/ilmath]
• We have shown [ilmath]\exists\epsilon >0[x+\epsilon<y][/ilmath]

This completes this part of the proof.

TODO: Fix warning. Note that [ilmath]x+\epsilon < y\implies x+\epsilon \le y[/ilmath] so this content isn't wrong, but it requires multiplication by [ilmath]\frac{1}{2} [/ilmath] which you cannot do in the ring [ilmath]\mathbb{Z} [/ilmath] for example.