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

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

Offset

1,3

Comments

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

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

Links

Table of n, a(n) for n=1..11.

Makoto Kamada, Prime numbers of the form 200...009.

Sabin Tabirca and Kieran Reynolds, Lacunary Prime Numbers.

Formula

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

Example

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

Mathematica

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

Prog

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

Crossrefs

Cf. A081677, A088275, A101952.

Sequence in context: A072324 A204544 A082026 * A078260 A007185 A175852

Adjacent sequences: A101389 A101390 A101391 * A101393 A101394 A101395

Keyword

hard,nonn

Author

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

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

Status

approved