A274456 Numbers k such that 5*10^k + 77 is prime.

1, 2, 3, 4, 6, 8, 19, 27, 37, 56, 66, 136, 148, 387, 534, 536, 1273, 1593, 1796, 2026, 2164, 2502, 6128, 18714, 23327, 25427, 46461, 88182, 88377, 104326, 127153, 135019

Offset: 1

Author

Robert Price, Jun 23 2016

Keywords

• nonn
• more

Comments

For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 0 followed by the digits 77 is prime (see Example section).

a(33) > 2*10^5.

Examples

			3 is in this sequence because 5*10^3 + 77 = 5077 is prime.
Initial terms and associated primes:
a(1) = 1, 127;
a(2) = 2, 577;
a(3) = 3, 5077;
a(4) = 4, 50077;
a(5) = 6, 5000077, etc.
		

Links

• Makoto Kamada, Factorization of near-repdigit-related numbers.
• Makoto Kamada, Search for 50w77.

Crossrefs

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

Programs

Mathematica

Select[Range[0, 100000], PrimeQ[5*10^# + 77] &]

PARI

is(n)=ispseudoprime(5*10^n + 77) \\ Charles R Greathouse IV, Jun 13 2017

Extensions

a(30)-a(32) from Robert Price, Dec 30 2018