I'm reading a paper and I came across this notation, any idea? I looked for this notation but couldn't find much anywhere, at first sight I thought it is L infinity norm, but it seems it isn't that as well. $$ f(g)=\infty\Bbb{1}[{\Vert{w_g}\Vert}_0>K] $$ Note that the '1' above was in this font: $$ \Bbb{R} $$ However, I couldn't make this font work for numbers using the guide here: MathJax basic tutorial and quick reference
In case reference is required: https://arxiv.org/pdf/1602.04938.pdf
You can find this on the 4th page in this paper, 4 lines below equation (2).
Since the statement $\Vert w_g\Vert_0>K$ is required to be false when we restrict to at most $K$ words in the bag, the most likely meaning is that $1[p]$ is the Iverson bracket of a proposition $p$, normally denoted $[p]$, and the factor of $\infty$ changes the bracket from $0$ or $1$ to a loss function of $0$ or $\infty$.