The three dots mean "and so on" for Y and "and so on up to" for X. A set need not contain any elements. Although it may seem peculiar to consider sets without elements, these kinds of sets occur surprisingly often and in a variety of settings. For example, if s is the set of real number solutions of the equation x2 1 0, then S contains no elements. There is only one set that contains no elements, and it is called the empty set or sometimes the null set or void set). The empty set is denoted by 0. We also write (1. In addition to the example given above, the set of all real numbers x such that 0 is also empty. The elements of a set may in fact be sets themselves. The symbol below indicates the conclusion of an example.