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I saw this notation for $\in$ -- I'm assuming this means "defines", right?
$$\hat{\theta} \in \underset{\theta}{\operatorname{argmin}} \dfrac{1}{2} \sum_{j=1}^n (\theta^T x^j - y^j)^2$$
Does anyone have a reference for a description of the $\in$ notation used here?
Technically $\arg \min_{x \in A} f(x)$ is defined to be the set of all the minimizers of $f$ over $A$, even though in many of the cases we care about it is actually a singleton. So "$\in$" in that line is just set membership as usual.
