Infinite expression (mathematics)

In mathematics, an infinite expression is an expression in which some operators take an infinite number of arguments, or in which the nesting of the operators continues to an infinite depth.^[1] A generic concept for infinite expression can lead to ill-defined or self-inconsistent constructions (much like a set of all sets), but there are several instances of infinite expressions that are well defined.

Examples of well-defined infinite expressions include^[2]^[3] infinite sums, whether expressed using summation notation or as an infinite series, such as

\sum _{{n=0}}^{\infty }a_{n}=a_{0}+a_{1}+a_{2}+\cdots \,;

infinite products, whether expressed using product notation or expanded, such as

\prod _{{n=0}}^{\infty }b_{n}=b_{0}\times b_{1}\times b_{2}\times \cdots \,

infinite nested radicals, such as

{\sqrt {1+2{\sqrt {1+3{\sqrt {1+\cdots }}}}}}\,

infinite power towers, such as

{\sqrt {2}}^{{{\sqrt {2}}^{{{\sqrt {2}}^{{\cdot ^{{\cdot ^{{\cdot }}}}}}}}}}\,

and infinite continued fractions, whether expressed using Gauss's Kettenbruch notation or expanded, such as

c_{0}+{\underset {n=1}{{\overset {\infty }{{\mathrm K}}}}}\,{\frac {1}{c_{n}}}=c_{0}+{\cfrac {1}{c_{1}+{\cfrac {1}{c_{2}+{\cfrac {1}{c_{3}+{\cfrac {1}{c_{4}+\ddots }}}}}}}}\,

In infinitary logic, one can use infinite conjunctions and infinite disjunctions.

Even for well-defined infinite expressions, the value of the infinite expression may be ambiguous or not well defined; for instance, there are multiple summation rules available for assigning values to series, and the same series may have different values according to different summation rules if the series is not absolutely convergent.

From the hyperreal viewpoint

From the point of view of the hyperreals, such an infinite expression $E_\infty$ is obtained in every case from the sequence $\langle E_{n}:n\in {\mathbb {N}}\rangle$ of finite expressions, by evaluating the sequence at a hypernatural value $n=H$ of the index n, and applying the standard part, so that $E_{\infty }={\text{st}}(E_{H})$ .

References

↑ Helmer, Olaf (January 1938). "The syntax of a language with infinite expressions". Bulletin of the American Mathematical Society (Abstract). 44 (1): 33–34. doi:10.1090/S0002-9904-1938-06672-4. ISSN 0002-9904. OCLC 5797393. .
↑ Euler, Leonhard (November 1, 1988). Introduction to Analysis of the Infinite, Book I (Hardcover). J.D. Blanton (translator). Springer Verlag. p. 303. ISBN 978-0-387-96824-7.
↑ Wall, Hubert Stanley (March 28, 2000). Analytic Theory of Continued Fractions (Hardcover). American Mathematical Society. p. 14. ISBN 978-0-8218-2106-0.

This article is issued from Wikipedia - version of the 10/18/2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.

Infinite expression (mathematics)

From the hyperreal viewpoint

See also

References