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A294945 Numbers k such that 6*10^k - 91 is prime. 0
2, 6, 25, 30, 45, 98, 296, 498, 627, 2518, 3208, 3753, 4130, 7159, 7660, 10840, 16187, 24744, 28345, 1001, 1995, 1998, 31499, 32999, 44838, 60959, 88938 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For k > 1, numbers such that the digit 5 followed by k-2 occurrences of the digit 9 followed by the digits 09 is prime (see Example section).

a(28) > 2*10^5.

LINKS

Table of n, a(n) for n=1..27.

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

Makoto Kamada, Search for 59w09.

EXAMPLE

2 is in this sequence because 6*10^2 - 91 = 509 is prime.

Initial terms and primes associated:

a(1) = 2, 509;

a(2) = 6, 5999909;

a(3) = 25, 59999999999999999999999909;

a(4) = 30, 5999999999999999999999999999909;

a(5) = 45, 5999999999999999999999999999999999999999999909; etc.

MATHEMATICA

Select[Range[2, 100000], PrimeQ[6*10^# - 91] &]

PROG

(PARI) isok(k) = isprime(6*10^k - 91); \\ Michel Marcus, Nov 12 2017

CROSSREFS

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

Sequence in context: A123150 A086591 A173609 * A073545 A103063 A220191

Adjacent sequences:  A294942 A294943 A294944 * A294946 A294947 A294948

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Nov 11 2017

STATUS

approved

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Last modified November 22 13:47 EST 2019. Contains 329393 sequences. (Running on oeis4.)