Index: The Book of Statistical Proofs ▷ Probability Distributions ▷ Univariate continuous distributions ▷ Gamma distribution ▷ Expectation of x ln x

Theorem: Let $X$ be a random variable following a gamma distribution:

\[\label{eq:gam} X \sim \mathrm{Gam}(a, b) \; .\]

Then, the mean or expected value of $(X \cdot \ln X)$ is

\[\label{eq:gam-xlogx} \mathrm{E}(X \ln X) = \frac{a}{b} \left[ \psi(a) - \ln(b) \right] \; .\]

Proof: With the definition of the expected value, the law of the unconscious statistician and the probability density function of the gamma distribution, we have:

\[\label{eq:gam-xlogx-s1} \begin{split} \mathrm{E}(X \ln X) &= \int_{0}^{\infty} x \ln x \cdot \frac{b^a}{\Gamma(a)} x^{a-1} \exp[-b x] \, \mathrm{d}x \\ &= \frac{1}{\Gamma(a)} \int_{0}^{\infty} \ln x \cdot \frac{b^{a+1}}{b} x^{a} \exp[-b x] \, \mathrm{d}x \\ &= \frac{\Gamma(a+1)}{\Gamma(a) \, b} \int_{0}^{\infty} \ln x \cdot \frac{b^{a+1}}{\Gamma(a+1)} x^{(a+1)-1} \exp[-b x] \, \mathrm{d}x \\ \end{split}\]

The integral now corresponds to the logarithmic expectation of a gamma distribution with shape $a+1$ and rate $b$

\[\label{eq:logmean-a+1} \mathrm{E}(\ln Y) \quad \text{where} \quad Y \sim \mathrm{Gam}(a+1,b)\]

which is given by

\[\label{eq:gam-logmean} \mathrm{E}(\ln Y) = \psi(a+1) - \ln(b)\]

where $\psi(x)$ is the digamma function. Additionally employing the relation

\[\label{eq:gam-fct} \Gamma(x+1) = \Gamma(x) \cdot x \quad \Leftrightarrow \quad \frac{\Gamma(x+1)}{\Gamma(x)} = x \; ,\]

the expression in equation \eqref{eq:gam-xlogx-s1} develops into:

\[\label{eq:gam-xlogx-qed} \mathrm{E}(X \ln X) = \frac{a}{b} \left[ \psi(a) - \ln(b) \right] \; .\]
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Metadata: ID: P179 | shortcut: gam-xlogx | author: JoramSoch | date: 2020-10-15, 13:02.