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20 (another Prime Pages' Curiosity) |
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The smallest composite number r such that n = 4r + 3 and m = 8r + 7 are primes. [Rivera] Oliver Sacks' describes in his book 'The MAN Who MISTOOK HIS WIFE for a HAT,' a pair of autistic twin brothers who were swapping 20-figure primes. The smallest number n such that neither 6n-1 nor 6n+1 are primes. [Necula] 20 ones plus 20 is prime. This is the largest such two-digit number. [Opao] There are 20 books in the Bible that have a prime number of chapters. [Opao] 20^20 - 11...1(20 ones) & 20 + 11...1(20 ones) are primes. [Firoozbakht] The 20th palindromic prime contains a digit sum of 20. [Post]
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