A295967 Numbers k such that (35*10^k - 377)/9 is prime.

2, 3, 6, 24, 33, 86, 335, 711, 1103, 1169, 1181, 2163, 3026, 7940, 8174, 9365, 10127, 13466, 40130, 70130, 174704, 198294

Offset

1,1

Comments

For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 8 followed by the digits 47 is prime (see Example section).

a(23) > 2*10^5.

Links

Table of n, a(n) for n=1..22.

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

Makoto Kamada, Search for 38w47.

Example

2 is in this sequence because (35*10^2 - 377)/9 = 347 is prime.
Initial terms and associated primes:
a(1) = 2, 347;
a(2) = 3, 3847;
a(3) = 6, 3888847;
a(4) = 24, 3888888888888888888888847;
a(5) = 33, 3888888888888888888888888888888847; etc.

Mathematica

Select[Range[2, 100000], PrimeQ[(35*10^# - 377)/9] &]

Prog

(PARI) isok(k) = isprime((35*10^k - 377)/9); \\ Michel Marcus, Dec 01 2017

Crossrefs

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

Sequence in context: A327013 A171708 A329565 * A027163 A160683 A377183

Adjacent sequences: A295964 A295965 A295966 * A295968 A295969 A295970

Keyword

nonn,more,hard

Author

Robert Price, Nov 30 2017

Extensions

a(21)-a(22) from Robert Price, Jul 19 2018

Status

approved