Moment Generating Function of Geometric Distribution/Formulation 1/Examples/Third Moment

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Examples of Use of Moment Generating Function of Geometric Distribution/Formulation 1

Let $X \sim \Geometric p$ for some $0 < p < 1$, where $\Geometric p$ is the Geometric distribution.

$\map X \Omega = \set {0, 1, 2, \ldots} = \N$
$\map \Pr {X = k} = \paren {1 - p} p^k$


The third moment generating function of $X$ is given by:

$\map { {M_X}'''} t = p \paren {1 - p } e^t \paren {\dfrac {1 + 4p e^t + p^2 e^{2t} } {\paren {1 - p e^t}^4 } }$


Proof

We have:

\(\ds \map { {M_X}'''} t\) \(=\) \(\ds \frac \d {\d t} \map { {M_X}''} t\) Definition of Moment Generating Function
\(\ds \) \(=\) \(\ds \frac \d {\d t} p \paren {1 - p} \paren {\dfrac {e^t + p e^{2t} } {\paren {1 - p e^t}^3 } }\) Moment Generating Function of Geometric Distribution: Second Moment
\(\ds \) \(=\) \(\ds p \paren {1 - p} \frac \d {\d t} \paren {e^t + p e^{2t} } \paren {1 - p e^t}^{-3}\) factoring out the $p \paren {1 - p}$
\(\ds \) \(=\) \(\ds p \paren {1 - p } \paren {\paren {e^t + 2p e^{2t} } \paren {1 - p e^t}^{-3} + \paren {e^t + p e^{2t} } \paren {-3 \paren {1 - p e^t}^{-4} } \paren {-p e^t } }\) Product Rule, Chain Rule for Derivatives, Derivative of Power, Derivative of Exponential Function
\(\ds \) \(=\) \(\ds p \paren {1 - p } \paren {\dfrac {e^t + 2p e^{2t} } {\paren {1 - p e^t}^3 } + \dfrac {3p e^{2t} + 3p^2 e^{3t} } {\paren {1 - p e^t}^4 } }\) gathering terms
\(\ds \) \(=\) \(\ds p \paren {1 - p } \paren {\dfrac {e^t + 2p e^{2t} } {\paren {1 - p e^t}^3 } \dfrac {\paren {1 - p e^t} } {\paren {1 - p e^t} } + \dfrac {3p e^{2t} + 3p^2 e^{3t} } {\paren {1 - p e^t}^4 } }\) multiplying by $1$
\(\ds \) \(=\) \(\ds p \paren {1 - p } \dfrac {e^t + 2p e^{2t} - p e^{2t} - 2p^2 e^{3t} + 3p e^{2t} + 3p^2 e^{3t} } {\paren {1 - p e^t}^4 }\) simplifying
\(\ds \) \(=\) \(\ds p \paren {1 - p } e^t \paren {\dfrac {1 + 4p e^t + p^2 e^{2t} } {\paren {1 - p e^t}^4 } }\) simplifying

$\blacksquare$