13

This number is a prime.

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Photo credit: Patrick Kik

The number (coincidentally prime) of known positive integers which are not the sum of two semiprimes. [Capelle]

The only known multidigit number n such that sigma(n) is the reversal of prime(n). [Capelle]

The number of brilliant Harshad (or Niven) numbers in base 10. [Capelle]

There are 13 positive numbers n such that n is equal to the number of 2's in the decimal digits of all natural numbers smaller or equal to n. [Capelle]

The smallest prime number having exactly one representation as sum of squares greater than one. Note that its reversal is the largest one. [Capelle]

There are exactly 13 positive integers that cannot be written as the sum of four hexagonal numbers. Their sum is a prime number whose sum of digits is 13. [Capelle]