19

This number is a prime.

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If p, q, r, s are consecutive prime numbers such that p > q > r > s > 19, then floor((p * q)/(r * s)) = 1. [Capelle]

There are exactly 19 primes beginning with the digit 2 among primes smaller than one thousand. [Capelle]

The smallest multidigit number (coincidentally prime) that does not form a palindrome when added to its reversal. [Capelle]

Farideh Firoozbakht improved the Papadimitriou's conjecture and proposed that every prime number p greater than or equal to 19 can be written as p = 2q + 3r, where q and r are odd primes. [Capelle]

The largest natural number n for which the sum of digits is not smaller than n/2. [Capelle]