## Involve: A Journal of Mathematics

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

### Infinite sums in totally ordered abelian groups

Greg Oman, Caitlin Randall, and Logan Robinson

#### 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**

Received: 4 August 2017

Revised: 19 January 2018

Accepted: 22 January 2018

First available in Project Euclid: 25 October 2018

**Permanent link to this document**

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

**Subjects**

Primary: 03E10: Ordinal and cardinal numbers 06F20: Ordered abelian groups, Riesz groups, ordered linear spaces [See also 46A40]

Secondary: 40J99: None of the above, but in this section

**Keywords**

infinite series totally ordered abelian group transfinite induction transfinite recursion well-ordered set

#### 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