What prime number comes next after 19?

The story behind the answer

The next prime after 19 is 23. Among the next odd numbers: 21 = 3×7 (composite), 23 has no divisors other than 1 and itself (not divisible by 2, 3, or 5), 25 = 5×5, and 27 = 3×9. So 23 is the first prime you meet.

Tip: To test primality quickly, try dividing by primes up to the square root of the number and use simple rules—evens aren’t prime (except 2), multiples of 3 have digits summing to a multiple of 3, and multiples of 5 end in 0 or 5. Primes are the atoms of integers via unique prime factorization.