A101395 Numbers k such that 4*10^k+7 is prime.

0, 1, 3, 9, 39, 2323

Offset: 1

Author

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

Keywords

• hard
• nonn

Comments

No further terms < 50000.

a(7) > 2*10^5. - Robert Price May 16 2015

Examples

			n = 1, 3, 9 are members since 47, 4007 and 4000000007 are primes.
		

Links

• Makoto Kamada, Prime numbers of the form 400...007.
• Sabin Tabirca and Kieran Reynolds, Lacunary Prime Numbers.
• Index entries for primes involving repunits.

Crossrefs

Cf. A056806, A100501, A088274, A101714.

Programs

Mathematica

Do[ If[ PrimeQ[4*10^n + 7], Print[n]], {n, 0, 10000}]

PARI

is(n)=ispseudoprime(4*10^n+7) \\ Charles R Greathouse IV, Jun 12 2017

Formula

a(n) = A101714(n-1) + 1.