"Lucky numbers" happens to be a notion in mathematics, in number theory to be more exact. They are natural numbers that are left over from an elimination process, e.g. prime numbers are the numbers that remain after applying the "sieve of Erastothenes" (which is why they are "lucky").

What becomes interesting is to see which operations remain possible within the set of those lucky numbers, e.g. if you do a multiplation within the set of prime numbers, the result is not a prime number.

I am not an expert in number theory - it is all a bit long ago and far away by now - but I remember there was a set that also had a specific name (which I forgot) and where the numbers where all of the (3n + 1) type (n being any natural number). Operations like addition and substraction gave a result that was not a "lucky number", but by multiplication you got a result within the set.