What does it mean when a number is subscripted with a truth statement? I have seen the following in several papers: $1_{\lvert r\rvert>1}$. What does this mean? Does this evaluate to 1 if $\lvert r\rvert>1$ and 0 otherwise? What would this evaluate to if instead of 1 we had a variable like $x$?
Thanks.

Yes, it is an indicator function: $1$ when the condition is true, $0$ when it is not.  I have not seen it used with $x$ instead of $1$.
