MHB An increasing function on the power set of a set

AI Thread Summary
The discussion centers on a function f defined on the power set of a finite set A, which maintains the property that if X is a subset of Y, then f(X) is a subset of f(Y). Participants explore the existence of a set T in the power set such that f(T) equals T. This property is linked to the Knaster–Tarski theorem, which asserts that such a fixed point T exists. The theorem's applicability extends beyond finite sets, making it a significant concept in set theory. Understanding this theorem enhances comprehension of increasing functions on power sets.
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Let $A$ be a finite set. Let $\mathcal{P}(A)$ denote the power set of $A$. Let $f:\mathcal{P}(A)\to \mathcal{P}(A)$ be a function such that $X\subseteq Y\Rightarrow f(X)\subseteq f(Y)$. Show that $\exists T\in \mathcal{P}(A)$ such that $f(T)=T$.

P.S. The power set of $A$ is the set of all the subsets of $A$.
NOTE: The theorem holds even when $A$ is not finite.
 
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Evgeny.Makarov said:
This is a special case of Knaster–Tarski theorem.
I didn't know about this theorem. Thanks.
 
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