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He helped to popularize the 0-9 digit number, came up with the Fibonacci sequence and the sub-set of Fibonacci primes.
The first few Fibonacci primes are 2, 3, 5, 13, 89. Fibonacci primes with thousands of digits have been found but it is not known whether there are infinitely many (Wikipedia, see link).
The set of primes would be one. The set of Mersenne primes is another. The set of all primes below 50 is another. And so on. A set which includes all primes, and only them, is the set of numbers having exactly 2 factors.
sub set is set
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