Definition:Basis (Topology)/Synthetic Basis

From ProofWiki
Jump to navigation Jump to search

Definition

Let $S$ be a set.

Definition 1

A synthetic basis on $S$ is a subset $\BB \subseteq \powerset S$ of the power set of $S$ such that:

\((\text B 1)\)   $:$   $\BB$ is a cover for $S$             
\((\text B 2)\)   $:$     \(\displaystyle \forall U, V \in \BB:\) $\exists \AA \subseteq \BB: U \cap V = \bigcup \AA$             

That is, the intersection of any pair of elements of $\BB$ is a union of sets of $\BB$.


Definition 2

A synthetic basis on $S$ is a subset $\mathcal B \subseteq \mathcal P \left({S}\right)$ of the power set of $S$ such that:

$\mathcal B$ is a cover for $S$
$\forall U, V \in \mathcal B: \forall x \in U \cap V: \exists W \in \mathcal B: x \in W \subseteq U \cap V$


Also see

  • Results about bases can be found here.


Linguistic Note

The plural of basis is bases.

This is properly pronounced bay-seez, not bay-siz.