Template:Sequence of the Day for July 11

Intended for: July 11, 2012

Timetable

• First draft entered by Alonso del Arte on April 20, 2011 ✓
• Draft reviewed by Daniel Forgues on July 9, 2011 ✓
• Draft approved by Alonso del Arte on July 10, 2011 ✓

Yesterday's SOTD * Tomorrow's SOTD

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A004022: Repunit primes in base 10.

{ 11, 1111111111111111111, 11111111111111111111111, ... }

These primes are of the form

10 n  −  1 9

. The exponent

n

must itself also be prime. Those three repunit primes have 2, 19 and 23 digits, respectively. The next repunit prime has 317 digits!