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A294943 Numbers k such that (865*10^k - 1)/9 is prime. 0
7, 13, 25, 40, 58, 133, 229, 232, 310, 358, 778, 977, 1057, 1271, 4417, 4757, 5242, 6413, 11539, 60583, 149737 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For k > 1, numbers such that the digits 96 followed by k occurrences of the digit 1 is prime (see Example section).

a(22) > 2*10^5.

LINKS

Table of n, a(n) for n=1..21.

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

Makoto Kamada, Search for 961w.

EXAMPLE

7 is in this sequence because (865*10^7 - 1)/9 = 961111111 is prime.

Initial terms and primes associated:

a(1) = 7, 961111111;

a(2) = 12, 961111111111111;

a(3) = 23, 961111111111111111111111111;

a(4) = 40, 961111111111111111111111111111111111111111;  etc.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[(865*10^# - 1)/9] &]

PROG

(PARI) isok(k) = isprime((865*10^k - 1)/9); \\ Michel Marcus, Nov 12 2017

CROSSREFS

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

Sequence in context: A255653 A200270 A031887 * A111721 A213663 A060455

Adjacent sequences:  A294940 A294941 A294942 * A294944 A294945 A294946

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Nov 11 2017

EXTENSIONS

a(21) from Robert Price, Apr 18 2020

STATUS

approved

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Last modified July 15 04:34 EDT 2020. Contains 335763 sequences. (Running on oeis4.)