Mathematics is no different from others, it needs to collect objects into meaningful groups
Among such, groups of numbers we commonly use in mathematics are N = Collection of natural numbers 1, 2, 3.... W = Collection of whole numbers 0, 1, 2, 3..... I or Z = Collection of Integers 0, ± 1, ± 2, ± 3, ...... Q = Collection of rational numbers i.e the numbers that can be written in p/q formwhere p, q are integers and q is not equal to 0 ¡ = Collection of real numbers i.e. the numbers which have decimal expansion.However, we sometimes come across certain groups, in which the objects do not share any common property.
For example, 2, Ramesh, January There may be some reason for forming this group, but we don’t see any common property shared by these objects. Nevertheless, we are sure that there are only three objects in this group and we also know what these objects are. In other words, these objects are ‘well defined’.