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A281828 Numbers k such that 7*10^k - 89 is prime. 0
3, 4, 6, 7, 12, 13, 23, 28, 42, 51, 66, 247, 291, 623, 3429, 3892, 4416, 7366, 7991, 8737, 10151, 10209, 40129, 41604, 50818, 68088 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For k>1, numbers such that the digit 6 followed by k-2 occurrences of the digit 9 followed by the digits 11 is prime (see Example section).

a(27) > 10^5.

LINKS

Table of n, a(n) for n=1..26.

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

Makoto Kamada, Search for 69w11.

EXAMPLE

4 is in this sequence because 7*10^4 - 89 = 69911 is prime.

Initial terms and primes associated:

a(1) = 3, 6911;

a(2) = 4, 69911;

a(3) = 6, 6999911;

a(4) = 7, 69999911;

a(5) = 12, 6999999999911; etc.

MATHEMATICA

Select[Range[2, 100000], PrimeQ[7*10^# - 89] &]

CROSSREFS

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

Sequence in context: A033162 A105133 A211384 * A002982 A290432 A276783

Adjacent sequences:  A281825 A281826 A281827 * A281829 A281830 A281831

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Jan 31 2017

STATUS

approved

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Last modified February 15 16:25 EST 2019. Contains 320136 sequences. (Running on oeis4.)