# Integral of Positive Measurable Function is Monotone

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## Theorem

Let $\left({X, \Sigma, \mu}\right)$ be a measure space.

Let $f,g: X \to \overline{\R}$, $f,g \in \mathcal{M}_{\overline{\R}}^+$ be positive measurable functions.

Suppose that $f \le g$, where $\le$ denotes pointwise inequality.

Then:

- $\displaystyle \int f \, \mathrm d\mu \le \int g \, \mathrm d\mu$

where the integral sign denotes $\mu$-integration.

This can be summarized by saying that $\displaystyle \int \cdot \, \mathrm d\mu$ is monotone.

## Proof

## Sources

- 2005: René L. Schilling:
*Measures, Integrals and Martingales*... (previous) ... (next): $9.8 \ \text{(iv)}$, $\S 9$: Problem $2$