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25 is a strong pseudoprime base 2 + 5. [Gupta] The only prime square whose digits are all prime. The smallest twodigit 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] There are exactly 5^{2} squares less than or equal to 1000^{2} 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 = 5^{2}. [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+5digit prime. [Silva] A prime square whose prime count (5^2) is another prime square (3^2) and it's prime count is another prime square (((5^2)) = 2^2). [Picheny]
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