A275287 Numbers k such that (26*10^k - 107)/9 is prime.

1, 2, 5, 7, 13, 23, 25, 44, 110, 157, 751, 853, 964, 1135, 2588, 2602, 2807, 2888, 3272, 4046, 4798, 16720, 21410, 22285, 25888, 34586, 51764

Offset: 1

Author

Robert Price, Jul 21 2016

Keywords

• nonn
• more

Comments

For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 8 followed by the digits 77 is prime (see Example section).

a(28) > 10^5.

Examples

			5 is in this sequence because (26*10^5 - 107)/9 = 288877 is prime.
Initial terms and associated primes:
a(1) = 1, 17;
a(2) = 2, 277;
a(3) = 5, 288877;
a(4) = 7, 28888877;
a(5) = 13, 28888888888877, etc.
		

Links

• Makoto Kamada, Factorization of near-repdigit-related numbers.
• Makoto Kamada, Search for 28w77.

Crossrefs

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

Programs

Mathematica

Select[Range[0, 100000], PrimeQ[(26*10^# - 107)/9] &]

PARI

lista(nn) = for(n=1, nn, if(ispseudoprime((26*10^n-107)/9), print1(n, ", "))); \\ Altug Alkan, Jul 22 2016