Intersection of Semilattice Ideals is Ideal
Theorem
Let $\struct {S, \preceq}$ be a meet semilattice.
Let $I_1, I_2$ be ideals in $\struct {S, \preceq}$.
Then $I_1 \cap I_2$ is ideal in $\struct {S, \preceq}$
Set of Sets
Let $\struct {S, \preceq}$ be a bounded below join semilattice.
Let $\II$ be a set of ideals in $\struct {S, \preceq}$.
Then $\bigcap \II$ is an ideal in $\struct {S, \preceq}$.
Proof
Directed Subset
Let $x, y \in I_1 \cap I_2$.
By definition of intersection:
- $x, y \in I_1$ and $x, y \in I_2$
By definition of directed subset:
- $\exists z_1 \in I_1: x \preceq z_1 \land y \preceq z_1$
and
- $\exists z_2 \in I_2: x \preceq z_2 \land y \preceq z_2$
- $z_1 \wedge z_2 \preceq z_1$ and $z_1 \wedge z_2 \preceq z_2$
By definition of lower section:
- $z_1 \wedge z_2 \in I_1$ and $z_1 \wedge z_2 \in I_2$
By definition of intersection:
- $z_1 \wedge z_2 \in I_1 \cap I_2$
By definition of meet:
- $z_1 \wedge z_2 = \inf \set {z_1, z_2}$
By definition of infimum:
- $x \preceq z_1 \wedge z_2$ and $y \preceq z_1 \wedge z_2$
Thus by definition:
- $I_1 \cap I_2$ is directed.
$\Box$
Lower Section
Let $x, y \in S$ such that:
- $x \in I_1 \cap I_2$ and $y \preceq x$
By definition of intersection:
- $x \in I_1$ and $x \in I_2$
By definition of lower section:
- $y \in I_1$ and $y \in I_2$
By definition of intersection:
- $y \in I_1 \cap I_2$
Thus by definition:
- $I_1 \cap I_2$ is a lower section.
$\Box$
Non-Empty Set
By assumption:
- $I_1 \ne \O$ and $I_2 \ne \O$
By definition of non-empty:
- $\exists x: x \in I_1$ and $\exists y: y \in I_2$
- $x \wedge y \preceq x$ and $x \wedge y \preceq y$
By definition of lower section:
- $x \wedge y \in I_1$ and $x \wedge y \in I_2$
By definition of intersection:
- $x \wedge y \in I_1 \cap I_2$
Thus by definition:
- $I_1 \cap I_2$ is non-empty.
$\Box$
Thus by definition:
- $I_1 \cap I_2$ is ideal in $\struct {S, \preceq}$.
$\blacksquare$
Sources
- Mizar article YELLOW_2:40