## Involve: A Journal of Mathematics

• Involve
• Volume 12, Number 2 (2019), 281-300.

### Infinite sums in totally ordered abelian groups

#### Abstract

The notion of convergence is absolutely fundamental in the study of calculus. In particular, it enables one to define the sum of certain infinite sets of real numbers as the limit of a sequence of partial sums, thus obtaining so-called convergent series. Convergent series, of course, play an integral role in real analysis (and, more generally, functional analysis) and the theory of differential equations. An interesting textbook problem is to show that there is no canonical way to “sum” uncountably many positive real numbers to obtain a finite (i.e., real) value. Plenty of solutions to this problem, which make strong use of the completeness property of the real line, can be found both online and in textbooks. In this note, we show that there is a more general reason for the nonfiniteness of uncountable sums. In particular, we present a canonical definition of “convergent series”, valid in any totally ordered abelian group, which extends the usual definition encountered in elementary analysis. We prove that there are convergent real series of positive numbers indexed by an arbitrary countable well-ordered set and, moreover, that any convergent series in a totally ordered abelian group indexed by an arbitrary well-ordered set has but countably many nonzero terms.

#### Article information

Source
Involve, Volume 12, Number 2 (2019), 281-300.

Dates
Revised: 19 January 2018
Accepted: 22 January 2018
First available in Project Euclid: 25 October 2018

https://projecteuclid.org/euclid.involve/1540432918

Digital Object Identifier
doi:10.2140/involve.2019.12.281

Mathematical Reviews number (MathSciNet)
MR3864218

Zentralblatt MATH identifier
06980502

#### Citation

Oman, Greg; Randall, Caitlin; Robinson, Logan. Infinite sums in totally ordered abelian groups. Involve 12 (2019), no. 2, 281--300. doi:10.2140/involve.2019.12.281. https://projecteuclid.org/euclid.involve/1540432918

#### References

• R. Botto Mura and A. Rhemtulla, Orderable groups, Lecture Notes in Pure and Applied Mathematics 27, Marcel Dekker, New York, 1977.
• H. B. Enderton, A mathematical introduction to logic, Academic, New York, 1972.
• G. B. Folland, Real analysis: modern techniques and their applications, 2nd ed., John Wiley & Sons, New York, 1999.
• T. Jech, Set theory, Springer, 2003.
• J. R. Munkres, Topology: a first course, Prentice-Hall, Englewood Cliffs, NJ, 1975.
• W. Rudin, Real and complex analysis, 3rd ed., McGraw-Hill, New York, 1987.
• T. Tao, An introduction to measure theory, Graduate Studies in Mathematics 126, American Mathematical Society, Providence, RI, 2011.
• T. Tao, Analysis, I, 3rd ed., Texts and Readings in Mathematics 37, Hindustan Book Agency, New Delhi, 2014.