A293538 Numbers k such that (8*10^k + 43)/3 is prime.

0, 1, 2, 4, 5, 10, 20, 34, 43, 70, 85, 138, 205, 574, 1378, 1512, 1770, 2434, 3073, 3330, 29443, 76840, 122203, 142932, 176908

Offset

1,3

Comments

For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 6 followed by the digits 81 is prime (see Example section).

a(26) > 2*10^5.

Links

Table of n, a(n) for n=1..25.

Makoto Kamada, Factorization of near-repdigit-related numbers.

Makoto Kamada, Search for 26w81.

Example

2 is in this sequence because (8*10^2 + 43)/3 = 281 is prime.
Initial terms and associated primes:
a(1) = 0, 17;
a(2) = 1, 41;
a(3) = 2, 281;
a(4) = 4, 26681;
a(5) = 5, 266681; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(8*10^# + 43)/3] &]

Prog

(PARI) lista(nn) = for(n=0, nn, if(isprime((8*10^n + 43)/3), print1(n, ", "))) \\ Iain Fox, Oct 18 2017

Crossrefs

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

Sequence in context: A245478 A018530 A018554 * A114879 A018620 A018664

Adjacent sequences: A293535 A293536 A293537 * A293539 A293540 A293541

Keyword

nonn,more,hard

Author

Robert Price, Oct 11 2017

Extensions

a(23)-a(25) from Robert Price, Jun 09 2018

Status

approved