# Why isn't conditional convergence an issue when defining $\sigma$-additivity of signed measures

A signed measure $\mu$ has to satisfy $$\mu(\bigcup_{n=1}^\infty A_n)=\sum_{n=1}^\infty\mu(A_n)$$ for $A_n$ disjoint and measurable. Clearly the LHS does not depend on rearrangements of the sequence $(A_n)$, but that's not so clear (to me) for the RHS. What happens if you choose $A_n$ so that the RHS converges conditionally? Doesn't Riemann's rearrangement theorem lead to a problem?

Either I'm missing something silly or we just "define away" this possibility. I looked at the Wikipedia page and my probability book (Shiryaev) and neither addresses the issue.

• But since this has to be true (by definition) for any sequence $A_1,\dots,A_n\dots$, doesn't the definition preclude this? If there was such a possibility for some signed measure $\mu$, i.e. a way to rearrange the $A_i$'s into $A_{\sigma(i)}$'s to change the sum, then applying this condition to the (valid) sequence $A_{\sigma(1)},\dots,A_{\sigma(n)}\dots$ would contradict the requirement (so that $\mu$ doesn't satisfy the definition of a signed measure after all)... doesn't it? – Clement C. Aug 28 '16 at 0:46
• So I guess the converse of Riemann's theorem implies that the sums have to converge absolutely. – Funktorality Aug 28 '16 at 0:51
• Yes. See also this (Discussion after Definition 2.3) of Measure Theory: A First Course by Carlos S. Kubrusly. – Clement C. Aug 28 '16 at 0:51
• Is it a triviality that the signed measures we usually talk about (eg. $A\mapsto\int_Af$) don't have sequences of disjoint measurable sets that have conditionally convergent sums? – Funktorality Aug 28 '16 at 0:52
• Convergence (to the same value) of the right hand side for any rearrangement is part of the requirement. This issue does not occur for 'positive' measures since sum of non-negative numbers always converges when we include $\infty$ to our number system. Now like you said, Riemann's theorem implies that the series actually converges absolutely so that's an useful fact we can infer, not a contradiction. – Dilemian Aug 28 '16 at 0:54

By definition, for any $\mu$ that satisfies the definition of a signed measure, such a sum cannot be conditionally convergent. It has to be absolutely convergent, as otherwise — as you observe — the RHS would not be order-invariant by Riemann's rearrangement theorem, while the LHS is.
So if $\mu$ is a signed measure, then for any sequence of disjoint measurable sets $(A_n)_{n\in\mathbb{N}}$, we have that $\left\lvert \sum_{n=1}^\infty \mu(A_n)\right\rvert < \infty$ implies absolute convergence of the series $\sum_{n=1}^\infty \mu(A_n)$.