A281911 Numbers k such that (185*10^k + 7)/3 is prime.

1, 4, 5, 14, 65, 74, 131, 320, 336, 528, 2156, 2370, 2658, 2660, 4940, 6108, 7662, 17813, 20330, 22235, 24600, 26280, 40241, 43936, 72484, 75777, 89063, 166837

Offset

1,2

Comments

For k>0, numbers such that the digits 61 followed by k-1 occurrences of the digit 6 followed by the digit 9 is prime (see Example section).

a(29) > 2*10^5.

Links

Table of n, a(n) for n=1..28.

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

Makoto Kamada, Search for 616w9.

Example

5 is in this sequence because (185*10^5 + 7)/3 = 6166669 is prime.
Initial terms and primes associated:
a(1) = 1, 619;
a(2) = 4, 616669;
a(3) = 5, 6166669;
a(4) = 14, 6166666666666669; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(185*10^# + 7)/3] &]

Prog

(PARI) isok(k) = isprime((185*10^k + 7)/3); \\ Michel Marcus, Feb 04 2017

Crossrefs

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

Sequence in context: A182252 A041857 A041034 * A042535 A222489 A222501

Adjacent sequences: A281908 A281909 A281910 * A281912 A281913 A281914

Keyword

nonn,more,hard

Author

Robert Price, Feb 01 2017

Extensions

a(28) from Robert Price, Mar 20 2020

Status

approved