I'm reading a paper that refers to a set $\mathcal{N}$, without defining it. It's a CS paper so it's not complicated maths. Is this the set of natural numbers? I don't get why they're using this style of N over a blackboard N.
Thanks
I'm reading a paper that refers to a set $\mathcal{N}$, without defining it. It's a CS paper so it's not complicated maths. Is this the set of natural numbers? I don't get why they're using this style of N over a blackboard N. Thanks 

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In 'PRIMES is in P' (for example), yes these are the natural numbers. There are various authors that do not use $\mathbb{N}$, and more generally do not use blackboard bold at all in print. As I mentioned on a recent question JeanPierre Serre is on record against balckboard bold (except on black boards). Though, a more typical choice then is $\mathbf{N}$, so standard bold. I could imagine that in CS, in particular when complexity classes are around, one wishes to have more optical distance between capital letters denoting complexity classes and those denoting sets. But this is mainly a guess. 

