A295394 Numbers k such that (2*10^k - 179)/3 is prime.

2, 3, 4, 6, 9, 25, 162, 258, 457, 813, 1079, 1266, 2397, 4075, 15688, 18663, 21079, 21851, 40627, 54960

Offset

1,1

Comments

For k > 1, numbers k such that k-2 occurrences of the digit 6 followed by the digits 07 is prime (see Example section).

a(21) > 2*10^5.

Links

Table of n, a(n) for n=1..20.

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

Makoto Kamada, Search for 6w07.

Example

3 is in this sequence because (2*10^3 - 179)/3 = 607 is prime.
Initial terms and associated primes:
a(1) = 2, 7;
a(2) = 3, 607;
a(3) = 4, 6607;
a(4) = 6, 666607;
a(5) = 9, 666666607; etc.

Mathematica

Select[Range[2, 100000], PrimeQ[(2*10^# - 179)/3] &]

Prog

(PARI) isok(k) = isprime((2*10^k - 179)/3); \\ Michel Marcus, Nov 22 2017

Crossrefs

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

Sequence in context: A103481 A215285 A275173 * A081419 A108858 A294688

Adjacent sequences: A295391 A295392 A295393 * A295395 A295396 A295397

Keyword

nonn,more,hard

Author

Robert Price, Nov 21 2017

Status

approved