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A284427 Numbers k such that (206*10^k-17)/9 is prime. 0
1, 2, 4, 5, 11, 16, 23, 193, 217, 412, 413, 2662, 4388, 5383, 6391, 7847, 12698, 14369, 16358, 20345, 28483, 75073, 79268, 100189, 116687 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For k>1, numbers such that the digits 22 followed by k-1 occurrences of the digit 8 followed by the digit 7 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 228w7.

EXAMPLE

4 is in this sequence because (206*10^4-17)/9 = 228887 is prime.

Initial terms and primes associated:

a(1) = 1, 227;

a(2) = 2, 2287;

a(3) = 4, 228887;

a(4) = 5, 2288887;

a(5) = 11, 2288888888887; etc.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[(206*10^#-17)/9] &]

CROSSREFS

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

Sequence in context: A294434 A191289 A256570 * A089416 A056817 A192581

Adjacent sequences:  A284424 A284425 A284426 * A284428 A284429 A284430

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Mar 26 2017

EXTENSIONS

a(24)-a(25) from Robert Price, Jan 14 2020

STATUS

approved

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Last modified August 5 22:11 EDT 2020. Contains 336214 sequences. (Running on oeis4.)