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A295083 Numbers k such that (115*10^k - 7)/9 is prime. 0
1, 2, 14, 26, 53, 86, 133, 206, 245, 323, 362, 872, 4562, 12577, 14294, 18425, 21713, 27728, 87368 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For k > 0, numbers such that the digits 12 followed by k occurrences of the digit 7 is prime (see Example section).

a(20) > 10^5.

LINKS

Table of n, a(n) for n=1..19.

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

Makoto Kamada, Search for 127w

EXAMPLE

2 is in this sequence because (115*10^2 - 7)/9 = 1277 is prime.

Initial terms and primes associated:

a(1) = 1, 127;

a(2) = 2, 1277;

a(3) = 14, 1277777777777777;

a(4) = 26, 1277777777777777777777777777;  etc.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[(115*10^# - 7)/9] &]

Select[Range[87500], PrimeQ[FromDigits[PadRight[{1, 2}, #, 7]]]&]-2 (* Harvey P. Dale, May 27 2018 *)

PROG

(PARI) isok(k) = isprime((115*10^k - 7)/9); \\ Michel Marcus, Nov 14 2017

CROSSREFS

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

Sequence in context: A017545 A280268 A109080 * A180877 A032461 A072390

Adjacent sequences:  A295080 A295081 A295082 * A295084 A295085 A295086

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Nov 13 2017

STATUS

approved

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Last modified March 20 19:23 EDT 2019. Contains 321349 sequences. (Running on oeis4.)