Sets are generally denoted by Capital Letters: A, B, C, ... Elements in the set represent lowercase letters: a, b, c, ... a ∈ A - a is an element in set A e.g. 2 ∈ N, 1/3 ∈ W, a ∉ A - a is not an element in set A e.g. 2/3 ∉ N, square root of 7 ∉ C, ∅ - empty set (has no elements), A = B - sets A and B are equal (have the same elements), A ⊆ B - set A is contained in B, set A is a subset of B e.g. A = {1, 2}, B = {1, 2, 3}, A ⊆ B. There are two ways to define sets: 1. list every elements e.g. A = {1, 5, 8}, B = {c, d, k}, C = {sister, brother, aunt}, 2. write a formula describing features of elements only in this set, A = {x ∈ R: x*2} - sef of real numbers less than 2, B = {x ∈ C: x^2 = 4} - set of integer numbers such that their square power equals 4.