A274911 Numbers k such that 7*10^k + 87 is prime.

1, 2, 5, 6, 18, 23, 59, 86, 115, 119, 251, 365, 370, 447, 1672, 3076, 3973, 5611, 7687, 8824, 13026, 17141, 17971, 23346, 29138, 94373, 94563, 142189, 156956, 255167, 266731

Offset: 1

Author

Robert Price, Nov 11 2016

Keywords

• nonn
• more

Comments

For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 0 followed by the digits 87 is prime (see Example section).

a(32) > 3*10^5.

Examples

			5 is in this sequence because 7*10^5 + 87 = 700087 is prime.
Initial terms and associated primes:
a(1) = 1, 157;
a(2) = 2, 787;
a(3) = 5, 700087;
a(4) = 6, 7000087;
a(5) = 18, 7000000000000000087, etc.
		

Links

• Makoto Kamada, Factorization of near-repdigit-related numbers.
• Makoto Kamada, Search for 70w87.

Crossrefs

Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.

Programs

Mathematica

Select[Range[0, 100000], PrimeQ[7*10^# + 87] &]

PARI

is(n)=ispseudoprime(7*10^n + 87) \\ Charles R Greathouse IV, Jun 13 2017

Extensions

a(28)-a(29) from Robert Price, Jul 27 2019

a(30)-a(31) from Robert Price, May 31 2023