# 25

This number is a composite. 25 is a strong pseudoprime base 2 + 5. [Gupta] The only prime square whose digits are all prime. The smallest two-digit number N = ab such that the sum and absolute difference of its digits along with the concatenations of each in both directions are all prime. E.g., 5 - 2 = 3, 5 + 2 = 7, 37, and 73 are prime. [Heleen] π(100) = 25. There are exactly 52 squares less than or equal to 10002 that are the sum of twin prime pairs. [Honaker] 25 is the concatenation of 2 and 5 which are the only primes of the form 10n + 2 and 10n + 5. Note that 25 = 52. [Murthy] Laura Nickel, a high school student when she made international news by codiscovering the 25th Mersenne prime, is now Ariel Glenn. There are exactly 25 (a perfect square) primes beginning with the digit 1 among primes smaller than one thousand. It's more than for the other digits. [Capelle] The smallest composite number whose prime factors are written with its digits. [Capelle] 25^5+52^2 is a 2+5-digit prime. [Silva] A prime square whose prime count π(5^2) is another prime square (3^2) and it's prime count is another prime square (π(π(5^2)) = 2^2). [Picheny] The website Prime Rhyme provides 25 words that rhyme with prime. The only known odd number whose sum of aliquot divisors is a perfect number. [Gupta]

(There are 9 curios for this number that have not yet been approved by an editor.)