Set Theory
4.0 Subsets
4.0 Subsets
If each element of a set $A$ is also an element of set $B$, then $A$ is called a subset of $B$.
And it is denoted as $$A \subseteq B$$
$$\begin{equation} \begin{aligned} \forall \;x \in A\;\;if\;x \in B\;and\;n(A) \le n(B) \\ \Rightarrow A \subseteq B \\\end{aligned} \end{equation} $$
