nLab Phoa's principle

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Idea

Let LL be a 01-bounded semilattice, that is, a semilattice with an absorbing element. Phoa’s principle states that every endofunction f:LLf:L \to L is monotonic and the precomposition function hhi:L LL 𝟚h \mapsto h \circ i:L^L \to L^\mathbb{2} by the embedding i:𝟚Li:\mathbb{2} \hookrightarrow L of the booleans into LL is an embedding.

If LL is a distributive lattice, then Phoa’s principle is equivalent to the linear interpolation condition that for all endofunctions f:LLf:L \to L and elements xLx \in L:

f(x)=f()(xf())f(x) = f(\top) \wedge (x \vee f(\bot))

In Pos the category of posets and monotonic functions, Phoa’s principle holds for the boolean domain 𝟚\mathbb{2}. In synthetic Stone duality, Phoa’s principle holds for the type of open propositions Open\mathrm{Open}. In synthetic domain theory, Phoa’s principle holds for a dominance Ω\Omega.

Phoa’s principle is named after Wesley Phoa.

References

Last revised on June 22, 2025 at 15:46:37. See the history of this page for a list of all contributions to it.