A101392 Numbers k such that 2*10^k+9 is prime.

0, 1, 5, 25, 455, 761, 9205, 13561, 15955, 26669, 113941

Offset: 1

Author

Julien Peter Benney (jpbenney(AT)ftml.net), Jan 15 2005

Keywords

• hard
• nonn

Comments

761 and 9205 only probably prime. No others less than 10000.

a(12) > 2*10^5. - Robert Price, Jun 06 2015

Examples

			n = 1, 5 are members since 29 and 200009 are primes.
		

Links

• Makoto Kamada, Prime numbers of the form 200...009.
• Sabin Tabirca and Kieran Reynolds, Lacunary Prime Numbers.

Crossrefs

Cf. A081677, A088275, A101952.

Programs

Mathematica

Do[ If[ PrimeQ[2*10^n + 9], Print[n]], {n, 0, 10000}]

PARI

is(n)=ispseudoprime(2*10^n+9) \\ Charles R Greathouse IV, Jun 12 2017

Formula

a(n) = A101952(n-1) + 1 for n>1.

Extensions

a(8)-a(9) from Kamada link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008

a(10)-a(11) from Kamada data by Robert Price, Dec 13 2010

Prepended a(1)=0 by Robert Price, Jun 06 2015