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A294572 Numbers k such that 8*10^k - 79 is prime. 0
5, 6, 13, 28, 64, 199, 233, 364, 457, 695, 1259, 1267, 1339, 1962, 2887, 3897, 6547, 7881, 11629, 13777, 15229, 17956, 24497, 48196 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For k > 1, numbers such that the digit 7 followed by k-2 occurrences of the digit 9 followed by the digits 21 is prime (see Example section).

a(25) > 10^5.

LINKS

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

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

Makoto Kamada, Search for 79w21

EXAMPLE

5 is in this sequence because 8*10^5 - 79 = 799921 is prime.

Initial terms and primes associated:

a(1) = 5, 799921;

a(2) = 6, 7999921;

a(3) = 13, 79999999999921;

a(4) = 28, 79999999999999999999999999921; etc.

MATHEMATICA

Select[Range[1, 100000], PrimeQ[8*10^# - 79] &]

CROSSREFS

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

Sequence in context: A138937 A314291 A314292 * A084381 A289895 A266304

Adjacent sequences:  A294569 A294570 A294571 * A294573 A294574 A294575

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Nov 02 2017

STATUS

approved

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Last modified September 21 23:55 EDT 2019. Contains 327286 sequences. (Running on oeis4.)