Approaching central projections in AF-algebras

Abstract

Let $A$ be a unital AF-algebra whose Murray-von Neumann order of projections is a lattice. For any two equivalence classes $[p]$ and $[q]$ of projections we write $[p]⊑[q]$ iff for every primitive ideal $\mathfrak{p}$ of $A$ either $p/\mathfrak{p}⪯q/\mathfrak{p}⪯(1-q)/\mathfrak{p}$ or $p/\mathfrak{p}⪰q/\mathfrak{p}⪰(1-q)/\mathfrak{p}.$ We prove that $p$ is central iff $[p]$ is $⊑$-minimal iff $[p]$ is a characteristic element in $K_0(A)$. If, in addition, $A$ is liminary, then each extremal state of $K_0(A)$ is discrete, $K_0(A)$ has general comparability, and $A$ comes equipped with a centripetal transformation $[p]\mapsto [p{]}^{⅁}$ that moves $p$ towards the center. The number $n(p)$ of $⅁$-steps needed by $[p]$ to reach the center has the monotonicity property $[p]⊑[q]\Rightarrow n(p)\le n(q).$ Our proofs combine the $K_0$-theoretic version of Elliott's classification, the categorical equivalence $\Gamma$ between MV-algebras and unital $\ell$-groups, and Łoś ultraproduct theorem for first-order logic.