Begin with P={2}; then form,m, the sum of 1 with the product overall elements of P. Place the smallest prime factor of m into P and repeat.

Suppose p = {p1,p2,…,pr}, then m = 1+ p1p2p3…pr.

Example:

2 is prime and 2+1 = 3 is prime;

2 * 3 +1 = 7 is prime;

2 * 3 * 7 +1 = 43 is prime;

2 * 3 * 7 * 43 +1 = 1807 = 13 * 139, then 13 is the prime;

Thus the first 5 prime number found by the classical proof is {2,3,7,43,13}.

So how to use this proof to find the first 20 prime in Mathematica?

Thank you.