# Subbases of a Topology

Recall from the Bases of a Topology page that if $(X, \tau)$ is a topological space then a basis for the topology $\tau$ is a collection $\mathcal B$ of open sets such that every $U \in \tau$ can be written as a union of a subcollection of open sets from $\mathcal B$. In other words, for every $U \in \tau$ there exists a $\mathcal B^* \subseteq \mathcal B$ such that:

(1)We will now define a similar term known as a subbasis.

Definition: Let $(X, \tau)$ be a topological space. A collection $\mathcal S \subseteq \tau$ is called a Subbasis for $\tau$ if the collection of finite intersections of elements from $\mathcal S$ forms a basis of $\tau$, i.e. $\mathcal B_S = \{ U_1 \cap U_2 \cap ... \cap U_k : U_1, U_2, ..., U_k \in \mathcal S \}$ is a basis of $\tau$. |

For example, consider the topological space $(\mathbb{R}, \tau)$ where $\tau$ is the usual topology of open intervals in $\mathbb{R}$. Consider the following set of semi-infinite open intervals:

(2)Notice that for $a, b \in \mathbb{R}$ and $a < b$ we have that:

(3)For $a \geq b$ we have that $(-\infty, b) \cap (a, \infty) = \emptyset$. Therefore the collection of all finite intersctions of elements from $\mathcal S$ are either open intervals or the empty set. Recall that the collection of open intervals already forms a basis of the usual topology on $\mathbb{R}$. Hence $S$ is a subbasis of $\tau$ since $\mathcal B_S = \{ U_1 \cap U_2 \cap ... \cap U_k : U_1, U_2, ..., U_k \}$ is a basis of $\tau$.