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A291870 Numbers k such that (55*10^k + 89)/9 is prime. 0
1, 3, 4, 7, 10, 31, 39, 70, 84, 135, 154, 279, 282, 522, 660, 765, 1153, 1980, 5670, 8064, 20823, 34290, 79711, 109759 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For k>1, numbers such that the digit 6 followed by k-2 occurrences of the digit 1 followed by the digits 21 is prime (see Example section).

a(25) > 2*10^5.

LINKS

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

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

Makoto Kamada, Search for 61w21

EXAMPLE

3 is in this sequence because (55*10^3 + 89)/9 = 6121 is prime.

Initial terms and primes associated:

a(1) = 1, 71;

a(2) = 3, 6121;

a(3) = 4, 61121;

a(4) = 7, 61111121;

a(5) = 10, 61111111121; etc.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[(55*10^# + 89)/9] &]

CROSSREFS

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

Sequence in context: A032715 A293276 A291609 * A002887 A080034 A061447

Adjacent sequences:  A291867 A291868 A291869 * A291871 A291872 A291873

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Sep 04 2017

EXTENSIONS

a(24) from Robert Price, Mar 01 2019

STATUS

approved

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Last modified July 14 09:22 EDT 2020. Contains 335720 sequences. (Running on oeis4.)