A108327 Integers n such that 10^n-21 is a prime number.

2, 6, 32, 108, 408, 1286, 2268, 2328, 4284, 53558, 181182, 249010

Offset: 1

Author

Julien Peter Benney (jpbenney(AT)ftml.net), Jun 30 2005

Keywords

• more
• nonn

Comments

a(13) > 2.5*10^5. - Robert Price, Apr 12 2015

Examples

			6 is a member because 10^6-21 = 1000000-21 = 999979, which is prime.
		

Links

• Makoto Kamada, Near-repdigit numbers of the form AA...AABA.
• Makoto Kamada, Prime numbers of the form 99...9979.
• Index entries for primes involving repunits.

Crossrefs

Cf. A089675, A095714, A092767.

Extensions

Edited by Ray Chandler, Dec 23 2010

a(10)=53558 from Robert Price, Mar 03 2011

a(11)-a(12) from Kamada data by Robert Price, Apr 12 2015