19 is a prime number with an interesting property. If you put a 1, 3, 7, or 9 at the end, then you will get 4 more primes. 191, 193, 197, and 199 are all prime. What is the next number p>19 such that p, p1, p3, p7, p9 are all prime?

I am sure that this is obvious from the puzzle, but p1 refers to the concatenation of the number p with the digit 1. The definition is similar for p3, p7, and p9. For example, if p=19, then p1=191, p3=193, p7=197, p9=199. In ordinary math notation, they are 10*p+1, 10*p+3, 10*p+7, 10*p+9.

Posted by Math Man on 2025-05-11 08:02:58