A286434 Numbers k such that (2*10^k - 59)/3 is prime.

2, 3, 6, 9, 10, 14, 15, 34, 56, 138, 250, 350, 357, 374, 392, 1594, 4794, 5290, 6702, 11936, 22296, 55762, 55834, 96195

Offset

1,1

Comments

For k>1, numbers such that k-2 occurrences of the digit 6 followed by the digits 47 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 6w47.

Example

3 is in this sequence because (2*10^3 - 59)/3 = 647 is prime.
Initial terms and primes associated:
a(1) = 2, 47;
a(2) = 3, 647;
a(3) = 6, 666647;
a(4) = 9, 666666647;
a(5) = 10, 6666666647; etc.

Mathematica

Select[Range[2, 100000], PrimeQ[(2*10^# - 59)/3] &]

Prog

(PARI) isok(k) = isprime((2*10^k-59)/3); \\ Michel Marcus, Mar 06 2018

Crossrefs

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

Sequence in context: A268692 A089160 A214974 * A288930 A045588 A191178

Adjacent sequences: A286431 A286432 A286433 * A286435 A286436 A286437

Keyword

nonn,more,hard

Author

Robert Price, May 09 2017

Status

approved