Prime Curios!: 20

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]

The smallest composite number which, summed to all previous primes, yields a prime. [Silva]

(20) = 2^3 + 0^3. It is the smallest number with this property. [Kumar]

(There are 4 curios for this number that have not yet been approved by an editor.)

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