A295402 Numbers k such that (28*10^k + 701)/9 is prime.

1, 2, 4, 5, 14, 17, 19, 29, 40, 56, 62, 82, 104, 121, 181, 286, 1307, 2197, 5764, 37120, 91715

Offset

1,2

Comments

For k > 1, numbers such that the digit 3 followed by k-2 occurrences of the digit 1 followed by the digits 89 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 31w89

Example

2 is in this sequence because (28*10^2 + 701)/9 = 389 is prime.
Initial terms and primes associated:
a(1) = 1, 109;
a(2) = 2, 389;
a(3) = 4, 31189;
a(4) = 5, 311189;
a(5) = 14, 311111111111189; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(28*10^# + 701)/9] &]

Prog

(PARI) isok(k) = isprime((28*10^k + 701)/9); \\ Michel Marcus, Nov 22 2017

Crossrefs

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

Sequence in context: A367298 A039574 A182375 * A121410 A002959 A306919

Adjacent sequences: A295399 A295400 A295401 * A295403 A295404 A295405

Keyword

nonn,more,hard

Author

Robert Price, Nov 21 2017

Status

approved