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51 and its reversal (15) are each products of two Fermat primes. [Harari] (51) = 15. (x) = the number of primes less than or equal to x. [Gupta] 51 = 2 + 3 + 5 + 41 . It's the smallest number which can be written with all the digits from 1 to 5 (without repetition) as a sum of primes. Note that the highest digit (5) and the lowest digit (1) are the digits of 51. [Capelle] The first base not of the form n^x (where generalized repunits can be factored algebraically) for which there are no known generalized repunit primes.
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