1973 1979 1987 1993 1997 1999

They are all prime! Nice trick with picking ones that look like years. I fell for it. First I looked at the differences, then I thought "maybe it’s some sort of baseball trivia?" Then I looked at the sum of the digits, and then the differences of the sums of the digits. So far I got nowhere.

My next step was going to be to look at the prime factorization. Before that, I happened to look at the sums of the digits again and saw that none of them were divisible by 3. So the original numbers weren’t divisible by 3, and then I saw they weren’t divisible by any of the "obvious" primes (2, 3, 5) either. So I thought, maybe they are prime.

And sure enough they were… so the next prime is 2003.

Fun!