---
title: Signed Measures
parent: Measure Theory
grand_parent: Measure and Integration
nav_order: 10
---

# {{ page.title }}

{% definition Signed Measure %}
A *signed measure* on a σ-algebra $\mathcal{A}$ on a set $X$
is a mapping $\mu : \mathcal{A} \to [-\infty,\infty]$ such that
{: .mb-0 }

- $\mu(\varnothing) = 0$,
- either there is no $A \in \mathcal{A}$ with $\mu(A) = -\infty$
  or there is no $A \in \mathcal{A}$ with $\mu(A) = \infty$,
- for every sequence $(A_n)_{n \in \NN}$ of
  pairwise disjoint sets $A_n \in \mathcal{A}$
  {: .my-0 }

  $$
  \mu \bigg\lparen \bigcup_{n=1}^{\infty} A_n \! \bigg\rparen
  = \sum_{n=0}^{\infty} \mu(A_n).
  $$
{% enddefinition %}

{% definition Measure Space %}
A *measure space* is a triple $(X,\mathcal{A},\mu)$ of
a set $X$,
a σ-algebra $\mathcal{A}$ on $X$
and a measure $\mu$ on $\mathcal{A}$.
{% enddefinition %}