A295586 Numbers k such that (25*10^k - 331)/9 is prime.

2, 3, 5, 6, 9, 12, 14, 18, 50, 56, 197, 222, 323, 1256, 1869, 2690, 4461, 5828, 46866, 116601, 141366

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 41 is prime (see Example section).

a(22) > 2*10^5.

Links

Table of n, a(n) for n=1..21.

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

Makoto Kamada, Search for 27w41.

Example

2 is in this sequence because (25*10^2 - 331)/9 = 241 is prime.
Initial terms and associated primes:
a(1) = 2, 241;
a(2) = 3, 2741;
a(3) = 5, 277741;
a(4) = 6, 2777741;
a(5) = 9, 2777777741; etc.

Mathematica

Select[Range[2, 100000], PrimeQ[(25*10^# - 331)/9] &]

Prog

(PARI) isok(k) = isprime((25*10^k - 331)/9); \\ Michel Marcus, Nov 24 2017

Crossrefs

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

Sequence in context: A124866 A166247 A126785 * A283768 A212864 A026317

Adjacent sequences: A295583 A295584 A295585 * A295587 A295588 A295589

Keyword

nonn,more,hard

Author

Robert Price, Nov 24 2017

Extensions

a(20)-a(21) from Robert Price, Aug 22 2018

Status

approved