diff --git a/I/Table_of_Contents.md b/I/Table_of_Contents.md index 36e53678..21b72f2d 100644 --- a/I/Table_of_Contents.md +++ b/I/Table_of_Contents.md @@ -58,6 +58,7 @@ Proofs by **[Number](/I/Proof_by_Number.html)** and **[Topic](/I/Proof_by_Topic. 3.1. Continuous uniform distribution
   3.1.1. *[Definition](/D/cuni.html)*
+    3.1.2. **[Probability density function](/P/cuni-pdf.html)**
3.2. Normal distribution
   3.2.1. *[Definition](/D/norm.html)*
diff --git a/P/cuni-pdf.md b/P/cuni-pdf.md new file mode 100644 index 00000000..17814e64 --- /dev/null +++ b/P/cuni-pdf.md @@ -0,0 +1,70 @@ +--- +layout: proof +mathjax: true + +author: "Joram Soch" +affiliation: "BCCN Berlin" +e_mail: "joram.soch@bccn-berlin.de" +date: 2020-01-31 15:41:00 + +title: "Probability density function of the continuous uniform distribution" +chapter: "Probability Distributions" +section: "Univariate continuous distributions" +topic: "Continuous uniform distribution" +theorem: "Probability density function" + +sources: + +proof_id: "P37" +shortcut: "cuni-pdf" +username: "JoramSoch" +--- + + +**Theorem:** Let $X$ be a random variable following a continuous uniform distribution: + +$$ \label{eq:cuni} +X \sim \mathcal{U}(a, b) \; . +$$ + +Then, the probability density function of $X$ is + +$$ \label{eq:cuni-pdf} +f_X(x) = +\begin{cases} +\frac{1}{b-a} & , \text{if} \; a \leq x \leq b \\ +\;\; 0 & , \text{otherwise} \; . +\end{cases} +$$ + + +**Proof:** A [continuous uniform variable is defined as](/D/cuni.html) having a constant probability density between minimum $a$ and maximum $b$. Therefore, + +$$ \label{eq:cuni-pdf-s1} +\begin{split} +f_X(x) &\propto 1 \quad \text{for all} \quad x \in [a,b] \quad \text{and} \\ +f_X(x) &= 0, \quad\!\! \text{if} \quad x < a \quad \text{or} \quad x > b \; . +\end{split} +$$ + +To ensure that $f_X(x)$ [is a proper probability density function](/D/pdf.html), the integral over all non-zero probabilities has to sum to $1$. Therefore, + +$$ \label{eq:cuni-pdf-s2} +f_X(x) = \frac{1}{c(a,b)} \quad \text{for all} \quad x \in [a,b] +$$ + +where the normalization factor $c(a,b)$ is specified, such that + +$$ \label{eq:cuni-pdf-s3} +\frac{1}{c(a,b)} \int_{a}^{b} 1 \, \mathrm{d}x = 1 \; . +$$ + +Solving this for $c(a,b)$, we obtain: + +$$ \label{eq:cuni-pdf-s4} +\begin{split} +\int_{a}^{b} 1 \, \mathrm{d}x &= c(a,b) \\ +[x]_a^b &= c(a,b) \\ +c(a,b) &= b-a \; . +\end{split} +$$ \ No newline at end of file