20 (another Prime Pages' Curiosity)
 Curios: Curios Search:   Participate: 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 5 curios for this number that have not yet been approved by an editor.)   To link to this page use http://primes.utm.edu/curios/page.php?number_id=388 Prime Curios! © 1999-2013 (all rights reserved)