I was reading this paper (equation 4 on page 2), and this equation had a strange notation that I've never seen before:

$$ g_r = g_q 1_{|r|\leq 1}$$

What does $1_{|r|\leq 1}$ mean? The author doesn't even try to explain this anywhere in the paper, which might suggest that it is common knowledge, but this is the first time I've seen something like this.

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    $\begingroup$ Looks like an indicator function. The value is 1 if $|r|\leq1$ and it is 0 otherwise. $\endgroup$ – Gerhard S. Oct 5 '17 at 5:55
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    $\begingroup$ My guess is that it's the characteristic function of the disk: 1 on the disk, 0 off. $\endgroup$ – NickD Oct 5 '17 at 5:57

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