6689

+ The smallest invertible prime whose digits are circular and composite. [Punches]

+ The NSW number with index 6689 is prime. These numbers arise when addressing the following question: "Is there a finite simple group whose order is a square?"

+ The first occurrence of a twin prime pair (6689, 6691) that both are invertible primes. [Beedassy]

+ An invertible prime equal to 2^7+3^8. [Silva]

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