Let $Z_1, Z_2, Z_3, \dots \in \text{Gaussian}(0, 1)$ each be a
D211: Standard gaussian random real number such that
A
D3161: Random real number $X \in \text{Random}(\mathbb{R})$ is a
chi-squared random real number with parameter $N \in \{ 1, 2, 3, \ldots \}$ if and only if
\begin{equation}
X
\overset{d}{=}
\sum_{n = 1}^N Z^2_n
\end{equation}
