Subsets

• A set A is a subset of a set B, A ⊆ B, if and only if all elements of A are also elements of B.
• A set A is a proper subset of set B, A ⊂B, if and only if A⊆ B and A≠B
• The empty set is a subset of all sets, and is a proper subset of all non-empty sets.
• if n(A) = i, there are 2 ⁱ subsets of A.
• if n(A) = i, there are 2ⁱ -1 proper subsets of A.