Definition:Upper Set

Definition

Let $\left({S, \preceq}\right)$ be an ordered set.

Let $U \subseteq S$.

Definition 1

$U$ is an upper set in $S$ if and only if:

$\forall u \in U: \forall s \in S: u \preceq s \implies s \in U$

Definition 2

$U$ is an upper set in $S$ if and only if:

$U^\succeq \subseteq U$

where $U^\succeq$ is the upper closure of $U$.

Definition 3

$U$ is an upper set in $S$ if and only if:

$U^\succeq = U$

where $U^\succeq$ is the upper closure of $U$.

Also known as

An upper set is also known as an upper-closed set or upward-closed set. Some sources call it an upset or up-set.

Also see

• Equivalence of Definitions of Upper Set

• Definition:Lower Set
• Definition:Upper Closure

• Results about upper sets can be found here.