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First four raw moments of a standard normal

Question

Let XX follow a standard normal distribution. Compute E[X]\mathbb{E}[X], E[X2]\mathbb{E}[X^2], E[X3]\mathbb{E}[X^3] and E[X4]\mathbb{E}[X^4].

Short answer

00, 11, 00 and 33. Odd moments vanish because the density is symmetric. The MGF et2/2=1+t22+t48+…e^{t^2/2}=1+\tfrac{t^2}{2}+\tfrac{t^4}{8}+\dots gives E[X2]=2!⋅12=1\mathbb{E}[X^2]=2!\cdot\tfrac12=1 and E[X4]=4!⋅18=3\mathbb{E}[X^4]=4!\cdot\tfrac18=3 (or integrate by parts: E[Xn]=(n−1)E[Xn−2]\mathbb{E}[X^{n}]=(n-1)\mathbb{E}[X^{n-2}]).

Adapted from Zhou, A Practical Guide to Quantitative Finance Interviews, sec. 4.4 Discrete and Continuous Distributions, 'Moments of normal distribution', p.90; question and answer rewritten by SelfMadeQuant.com.

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