If you interested, you can google "Lucky number". It will take you to Sieve of Eratosthenes. You can a little fun with it. It is still to be discovered whether there is infinite lucky numbers. You can write a computer program and find the lucky numbers.
You can also google Euler's Lucky Numbers and compare it. It says that someone has already proved that there are only six Euler's lucky numbers, but I didn't understand the material.
Apparently the proof for finite Euler Lucky Numbers is a corollary of The Stark-Heegner Theorem.