18731

This number is a prime.

The smallest prime that is the average of both its immediate and second neighbors. [Dale]

The smallest of six consecutive mountain primes that are all emirps. [Loungrides]

The largest prime in the smallest 3x3 magic square of emirps:

17207 18731 16493
16763 17477 18191
18461 16223 17747

Note that the magic constant is 52431, a number with digits 1-5 exactly once. [Gudipati]