A281646 Numbers k such that (25*10^k - 241)/9 is prime.

2, 4, 5, 8, 20, 35, 80, 131, 169, 262, 364, 484, 580, 692, 1687, 1834, 3476, 3950, 5963, 6329, 13339, 18617, 72703, 103360

Offset

1,1

Comments

For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 7 followed by the digits 51 is prime (see Example section).

a(25) > 2*10^5.

Links

Table of n, a(n) for n=1..24.

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

Makoto Kamada, Search for 27w51.

Example

4 is in this sequence because (25*10^4 - 241)/9 = 27751 is prime.
Initial terms and associated primes:
a(1) = 2, 251;
a(2) = 4, 27751;
a(3) = 5, 277751;
a(4) = 8, 277777751;
a(5) = 20, 277777777777777777751; etc.

Mathematica

Select[Range[1, 100000], PrimeQ[(25*10^# - 241)/9] &]

Prog

(PARI) isok(k) = ispseudoprime((25*10^k-241)/9); \\ Altug Alkan, May 07 2018

Crossrefs

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

Sequence in context: A253426 A092061 A256760 * A240460 A239405 A097684

Adjacent sequences: A281643 A281644 A281645 * A281647 A281648 A281649

Keyword

nonn,more,hard

Author

Robert Price, Jan 25 2017

Extensions

a(24) from Robert Price, May 07 2018

Status

approved