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A288821 Numbers k such that 10^k - 401 is prime. 0
3, 6, 9, 11, 12, 25, 47, 166, 222, 234, 352, 407, 515, 646, 1161, 2475, 4329, 7191, 12894, 16007, 25819, 67579, 88457, 117135 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For k>2, numbers such that k-3 occurrences of the digit 9 followed by the digits 599 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 9w599.

EXAMPLE

6 is in this sequence because 10^6 - 401 = 999599 is prime.

Initial terms and primes associated:

a(1) = 3, 599;

a(2) = 6, 999599;

a(3) = 9, 999999599;

a(4) = 11, 99999999599;

a(5) = 12, 999999999599; etc.

MATHEMATICA

Select[Range[3, 100000], PrimeQ[10^# - 401] &]

CROSSREFS

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

Sequence in context: A086883 A154777 A338432 * A343306 A267881 A310142

Adjacent sequences:  A288818 A288819 A288820 * A288822 A288823 A288824

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Jun 17 2017

EXTENSIONS

a(24) from Robert Price, Jan 19 2018

STATUS

approved

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Last modified June 22 04:06 EDT 2021. Contains 345367 sequences. (Running on oeis4.)