Let $X \in \text{Random}(\mathbb{R})$ be a D3161: Random real number such that

(i)	\begin{equation} \mathbb{E} |X|^2 < \infty \end{equation}

Then \begin{equation} \mathbb{E} X = \underset{\lambda \in \mathbb{R}}{\text{arg min}} \; \mathbb{E} |X - \lambda|^2 \end{equation}