In nonstandardNonstandard analysis deals primarily with the [[hyperreal number|hyperreal]] line,. The latter is an extension of the real line thatwhich allowscontains, in addition to the reals, foralso infinitely small quantities. In the hyperreal line every real number has a collection of numbers (called a [[monad (mathematics)|monad]]) of hyperreals infinitely close to it. The standard part function associates to a [[Wikt:finite|finite]] [[hyperreal number|hyperreal]] ''x'', the unique standard real number ''x<sub>0</sub>'' which infinitely close to it. InThe relationship is expressed symbolically by writting
