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]