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A289942 Numbers k such that (85*10^k - 1)/3 is prime. 0
1, 2, 5, 9, 64, 69, 118, 377, 409, 616, 959, 1093, 1726, 3034, 5929, 8140, 11197, 15293, 15381, 27811, 58091, 63757, 72025, 83921 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Numbers such that the digits 28 followed by k occurrences of the digit 3 is prime (see Example section).

a(25) > 10^5.

LINKS

Table of n, a(n) for n=1..24.

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

Makoto Kamada, Search for 283w.

EXAMPLE

2 is in this sequence because (85*10^2 - 1)/3 = 2833 is prime.

Initial terms and primes associated:

a(1) = 1, 283;

a(2) = 2, 2833;

a(3) = 5, 2833333;

a(4) = 9, 28333333333;

a(5) = 64, 283333333333333333333333333333333333333333333333333333333333333333; etc.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[(85*10^# - 1)/3] &]

PROG

(PARI) isok(k) = isprime((85*10^k - 1)/3); \\ Michel Marcus, Jul 16 2017

CROSSREFS

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

Sequence in context: A249583 A109469 A185160 * A239899 A188948 A083460

Adjacent sequences:  A289939 A289940 A289941 * A289943 A289944 A289945

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Jul 15 2017

STATUS

approved

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Last modified January 26 05:10 EST 2020. Contains 331273 sequences. (Running on oeis4.)