Standard part function


In nonstandard analysis, the standard part function is a function from the limited (finite) hyperreal numbers to the real numbers. Briefly, the standard part function "rounds off" a finite hyperreal to the nearest real. It associates to every such hyperreal , the unique real infinitely close to it, i.e. is infinitesimal. As such, it is a mathematical implementation of the historical concept of adequality introduced by Pierre de Fermat,[1] as well as Leibniz's Transcendental law of homogeneity.

The standard part function was first defined by Abraham Robinson who used the notation for the standard part of a hyperreal (see Robinson 1974). This concept plays a key role in defining the concepts of the calculus, such as continuity, the derivative, and the integral, in nonstandard analysis. The latter theory is a rigorous formalization of calculations with infinitesimals. The standard part of x is sometimes referred to as its shadow.[2]


The standard part function "rounds off" a finite hyperreal to the nearest real number. The "infinitesimal microscope" is used to view an infinitesimal neighborhood of a standard real.

Nonstandard analysis deals primarily with the pair  , where the hyperreals   are an ordered field extension of the reals  , and contain infinitesimals, in addition to the reals. In the hyperreal line every real number has a collection of numbers (called a monad, or halo) of hyperreals infinitely close to it. The standard part function associates to a finite hyperreal x, the unique standard real number x0 that is infinitely close to it. The relationship is expressed symbolically by writing


The standard part of any infinitesimal is 0. Thus if N is an infinite hypernatural, then 1/N is infinitesimal, and st(1/N) = 0.

If a hyperreal   is represented by a Cauchy sequence   in the ultrapower construction, then


More generally, each finite   defines a Dedekind cut on the subset   (via the total order on  ) and the corresponding real number is the standard part of u.

Not internal


The standard part function "st" is not defined by an internal set. There are several ways of explaining this. Perhaps the simplest is that its domain L, which is the collection of limited (i.e. finite) hyperreals, is not an internal set. Namely, since L is bounded (by any infinite hypernatural, for instance), L would have to have a least upper bound if L were internal, but L doesn't have a least upper bound. Alternatively, the range of "st" is  , which is not internal; in fact every internal set in   that is a subset of   is necessarily finite.[3]



All the traditional notions of calculus can be expressed in terms of the standard part function, as follows.



The standard part function is used to define the derivative of a function f. If f is a real function, and h is infinitesimal, and if f′(x) exists, then


Alternatively, if  , one takes an infinitesimal increment  , and computes the corresponding  . One forms the ratio  . The derivative is then defined as the standard part of the ratio:




Given a function   on  , one defines the integral   as the standard part of an infinite Riemann sum   when the value of   is taken to be infinitesimal, exploiting a hyperfinite partition of the interval [a,b].



Given a sequence  , its limit is defined by   where   is an infinite index. Here the limit is said to exist if the standard part is the same regardless of the infinite index chosen.



A real function   is continuous at a real point   if and only if the composition   is constant on the halo of  . See microcontinuity for more details.

See also



  1. ^ Katz, Karin Usadi; Katz, Mikhail G. (March 2012). "A Burgessian Critique of Nominalistic Tendencies in Contemporary Mathematics and its Historiography". Foundations of Science. 17 (1): 51–89. arXiv:1104.0375. doi:10.1007/s10699-011-9223-1The authors refer to the Fermat-Robinson standard part.{{cite journal}}: CS1 maint: postscript (link)
  2. ^ Bascelli, Tiziana; Bottazzi, Emanuele; Herzberg, Frederik; Kanovei, Vladimir; Katz, Karin U.; Katz, Mikhail G.; Nowik, Tahl; Sherry, David; Shnider, Steven (1 September 2014). "Fermat, Leibniz, Euler, and the Gang: The True History of the Concepts of Limit and Shadow" (PDF). Notices of the American Mathematical Society. 61 (08): 848. doi:10.1090/noti1149.
  3. ^ Goldblatt, Robert (1998). Lectures on the Hyperreals: An Introduction to Nonstandard Analysis. New York: Springer. ISBN 978-0-387-98464-3.

Further reading

  • H. Jerome Keisler. Elementary Calculus: An Infinitesimal Approach. First edition 1976; 2nd edition 1986. (This book is now out of print. The publisher has reverted the copyright to the author, who has made available the 2nd edition in .pdf format available for downloading at
  • Abraham Robinson. Non-standard analysis. Reprint of the second (1974) edition. With a foreword by Wilhelmus A. J. Luxemburg. Princeton Landmarks in Mathematics. Princeton University Press, Princeton, NJ, 1996. xx+293 pp. ISBN 0-691-04490-2