511

This number is a composite.

+ 5-1-1 = 3, 5*1*1 = 5, (5/1)/1 = 5, & 5+1+1 = 7. Note the curious symmetry among the odd primes and unity. [Capelle]

+ The 16th century French philosopher, Charles de Bouvelles, perhaps due to difficulties in division, cited falsely 511 as prime.

+ The smallest composite number that is the exact average of three consecutive primes (503, 509 and 521). [Gallardo]

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