In the equation:
$f_\theta(x)=\theta_1x$
Is there a reason that $\theta$ might be a subscript of $f$ and not either a second parameter or left out of the left side of the equation altogether? Does it differ from the following?
$f(x,\theta)=\theta_1x$
(I've been following the Machine Learning class and the instructor uses this notation that I've not seen before)
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$\begingroup$As you note, this is mostly notational choice. I might call the $\theta$ a parameter, rather than an independent variable. That is to say, you are meant to think of $\theta$ as being fixed, and $x$ as varying.
As an example (though I am not sure of the context you saw this notation), maybe you are interested in describing the collection of functions $$f(x) = x^2+c$$, where $c$ is a real number. I might call this function $f_c(x)$, so that I can later say that for $c\leq 0$, the function $f_c$ has two real roots, while for $c>0$ the roots are uniformly convex. I think these statements would be much more opaque if I made them about the function $f(x,c)$.
$\endgroup$ $\begingroup$I personally dislike the notation. And I believe it means different things in different contexts. For example, it means the opposite of what the other answers say in probability, see:
It is shown that $P_{\theta}(X = x)$ and $p_{\theta}(x)$ both mean that $X/x$ is the constant variable in this context and we are plotting with respect to $\theta$.
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