7

This number is a prime.

Just showing those entries submitted by 'Luhn': (Click here to show all)

Double 7! to get the exact number of minutes in a week (7 days). [Luhn]

The reversal of (7 - 1)! + 1 is 2⁷ - 1 (a Mersenne prime). [Luhn]

The first number of every sextuplet ends with 7. [Luhn]

The smallest example of x-y,xx-y,xxx-y all being prime is with digits x=7 and y=4. So 7-4=3, 77-4=73, 777-4=773 are all primes. It is the only solution because one of three consecutive numbers x..x-y is divisible by 3. [Luhn]