Can't figure out what this symbol in a Kac-Rice method means

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I've been reading a paper and I can't seem to understand what this cursive symbol "N" means at the beginning of the equation below. It's in the context of a staircase function and the Kac-Rice method. It's meant to be a function for the solutions between eigenvalues (if that helps). I've tried reaching out to the author but they haven't replied back yet. Is there a name for this symbol? Equation with unknown symbol Source of equation