# Proof

(related to Lemma: Zorn's Lemma)

• Let $(V,\preceq )$ be a poset.
• Assume, the Zorn's lemma is false.
• This means that although every chain $S\subseteq V$ has an upper bound, $V$ has no maximal element.
• Take $S_0:=\{a_0\}$ as an example of a chain in $V$ with $a_0\in V.$ * By assumption, $V$ contains at least one upper bound $u$ of $S_0$, i.e. $u\in V$ and $a_0\preceq u.$ * By assumption, none of the existing upper bounds $u$ can be maximal, i.e. there is at least one $x\in V$ with $x\succ u.$ * By the axiom of choice, we can choose $x$ from the set of all existing elements $x\in V$ with $x\succ u$ and set $a_1:=x.$ * By construction, $a_1\succ a_0$ and we can construct a new chain $S_1:=\{a_0,a_1\}.$
• By analogy, we can construct a new chain $S_3:=\{a_0,a_1,a_2\}$ with $a_0\prec a_1\prec a_2.$
• This process can be repeated "endlessly", and holds even for an infinite chain $S,$ while the axiom of choice ensures that we can extend $S$ by those elements of $X$ which are greater than any upper bound we have found for $S$ in $X.$
• But this contradicts the assumption that every chain $S$ has an upper bound in $X$ but still $X$ has no maximal elements.
• Therefore, the assumption is false and $X$ has at least one maximal element.

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### References

#### Bibliography

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3. Hoffmann, D.: "Forcing, Eine Einführung in die Mathematik der Unabhängigkeitsbeweise", Hoffmann, D., 2018