Subset

Let \(A\) and \(B\) be sets . We say that \(A\) is a subset of \(B\) provided every element of \(A\) is also a element of \(A\): that is \(A\) is a subset of \(B\) if, and only if for every \(a \in A\), we also have \(a \in B\). We write \(A \subseteq B\) to denote that \(A\) is a subset of \(B\).