7867

This number is a prime.

Write the prime numbers in order in a spiral on a square grid, with the 2 in the center, and draw lines outward from the 2 in the eight orthogonal and diagonal directions. In the limit a vanishingly small fraction of all primes lie on one of these eight lines, and 7867 is one of these rare primes. It is the 17th prime on the diagonal that starts on 2 and passes through 5. [Keith]