Set theory is the mathematical theory of well-determined collections, called sets, of objects that are called members, or elements, of the set. So, the essence of set theory is the study of infinite sets, and therefore it can be defined as the mathematical theory of the actual - as opposed to potential - infinite. The most basic properties are that a set can have elements and that two sets are equal (one and the same) if and only if every element of each set is an element of the other; this property is called the extensionality of sets. For example, the numbers 2, 4, and 6 are distinct objects when considered separately, but when they are considered collectively, they form a single set of size three, written {2,4,6}. A set is well-defined if there is no ambiguity as to whether or not an object belongs to it, i.e., a set is defined so that we can always tell what is and what is not a member of the set. Example: C = {red, blue, yellow, green, purple} is well-defined since it is clear what is in the set. Chapter 13 provides students with a clear idea of the basic concept, elements, representation, types of set and operations.