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A293278 Numbers k such that 9*10^k - 19 is prime. 0
1, 2, 5, 6, 7, 10, 11, 22, 24, 138, 140, 152, 698, 1180, 1270, 2970, 3932, 4972, 5982, 6287, 8936, 16014, 25186, 59862, 130998 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For k > 1, numbers such that the digit 8 followed by k-2 occurrences of the digit 9 followed by the digits 81 is prime (see Example section).

a(26) > 2*10^5.

LINKS

Table of n, a(n) for n=1..25.

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

Makoto Kamada, Search for 89w81.

EXAMPLE

5 is in this sequence because 9*10^5 - 19 = 899981 is prime.

Initial terms and primes associated:

a(1) = 1, 71;

a(2) = 2, 881;

a(3) = 5, 899981;

a(4) = 6, 8999981;

a(5) = 7, 89999981; etc.

MATHEMATICA

Select[Range[1, 100000], PrimeQ[9*10^# - 19] &]

CROSSREFS

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

Sequence in context: A037453 A014528 A286751 * A087791 A334880 A285032

Adjacent sequences:  A293275 A293276 A293277 * A293279 A293280 A293281

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Oct 04 2017

EXTENSIONS

a(25) from Robert Price, Jul 27 2019

STATUS

approved

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Last modified August 5 14:02 EDT 2021. Contains 346469 sequences. (Running on oeis4.)