Frink ideal

In mathematics, a Frink ideal, introduced by Orrin Frink, is a certain kind of subset of a partially ordered set.

Basic definitions

LU(A) is the set of all common lower bounds of the set of all common upper bounds of the subset A of a partially ordered set.

A subset I of a partially ordered set (P, ≤) is a Frink ideal, if the following condition holds:

For every finite subset S of I, we have LU(S) $\subseteq$ I.

A subset I of a partially ordered set (P, ≤) is a normal ideal or a cut if LU(I) $\subseteq$ I.

Every Frink ideal I is a lower set.
A subset I of a lattice (P, ≤) is a Frink ideal if and only if it is a lower set that is closed under finite joins (suprema).
Every normal ideal is a Frink ideal.

Frink, Orrin (1954). "Ideals in Partially Ordered Sets". American Mathematical Monthly. 61 (4): 223–234. doi:10.2307/2306387. JSTOR 2306387. MR 0061575.
Niederle, Josef (2006). "Ideals in ordered sets". Rendiconti del Circolo Matematico di Palermo. 55: 287–295. doi:10.1007/bf02874708. S2CID 121956714.