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A287678 Numbers k such that (62*10^k - 611)/9 is prime. 0
4, 7, 9, 10, 12, 13, 19, 40, 76, 87, 172, 175, 1005, 1330, 1582, 1849, 2134, 2350, 2652, 5680, 13504, 41416, 95251, 109233 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For k>1, numbers such that the digit 6 followed by k-2 occurrences of the digit 8 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 68w21.

EXAMPLE

4 is in this sequence because (62*10^4 - 611)/9 = 68821 is prime.

Initial terms and primes associated:

a(1) = 4, 68821;

a(2) = 7, 68888821;

a(3) = 9, 6888888821;

a(4) = 10, 68888888821;

a(5) = 12, 6888888888821; etc.

MATHEMATICA

Select[Range[1, 100000], PrimeQ[(62*10^# - 611)/9] &]

CROSSREFS

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

Sequence in context: A085746 A324599 A081828 * A310939 A175149 A153053

Adjacent sequences:  A287675 A287676 A287677 * A287679 A287680 A287681

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, May 29 2017

EXTENSIONS

a(24) from Robert Price, May 26 2019

STATUS

approved

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Last modified November 22 03:20 EST 2019. Contains 329383 sequences. (Running on oeis4.)