Carrier ideals, tail obstructions, and remainder traces for ladder-system spaces

Abstract

For a ladder-system space $X_L$ with carrier $S\subseteq E^{ω_1}_ω$, the finite-label uniformization property $M_{<ω}$ characterizes countable metacompactness, and countable metacompactness is equivalent to the $Δ$-property. Both equivalences are known for stationary carriers. For arbitrary carriers, an active-tail formulation gives a direct proof that $M_{<ω}$ is equivalent to the $Δ$-property and leads to a support-finite decomposition theorem, together with club-smallness and trace criteria that avoid explicit ladder-position thresholds. A club-gap argument, combined with Fodor's lemma, shows that finite and countable tail multiplicity determine the same carrier ideal, namely $\mathrm{NS}\restriction S$. Subsets of the isolated part that meet each ladder in only finitely many points have clopen remainder traces, and these traces form a generalized Boolean algebra. All such traces are disjoint from the carrier part of the remainder. Finally, the subcarriers whose restricted spaces are $σ$-closed discrete form an ideal $\mathcal{C}_L$ containing $\mathrm{NS}\restriction S$. If $X_L$ is a $Δ$-space, a threshold-based gluing argument shows that $\mathcal{C}_L$ is a $σ$-ideal. Whether this holds for every ladder system remains open.