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A295631 Numbers k such that (38*10^k + 187)/9 is prime. 0
2, 3, 5, 6, 9, 14, 17, 20, 54, 56, 165, 902, 1023, 6483, 14174, 18411, 20025, 27411, 49583, 59589, 66896, 97329 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For k > 1, numbers such that the digit 4 followed by k-2 occurrences of the digit 2 followed by the digits 43 is prime (see Example section).

a(23) > 2*10^5.

LINKS

Table of n, a(n) for n=1..22.

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

Makoto Kamada, Search for 42w43

EXAMPLE

2 is in this sequence because (38*10^2 + 187)/9 = 443 is prime.

Initial terms and primes associated:

a(1) = 2, 443;

a(2) = 3, 4243;

a(3) = 5, 422243;

a(4) = 6, 4222243;

a(5) = 9, 4222222243; etc.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[38*10^# + 187)/9] &]

Select[Range[2, 100000], PrimeQ[100*FromDigits[PadRight[{4}, #-1, 2]]+43]&] (* Harvey P. Dale, Jan 16 2018 *)

PROG

(PARI) is(k) = ispseudoprime((38*10^k + 187)/9) \\ Iain Fox, Nov 24 2017

CROSSREFS

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

Sequence in context: A175095 A073216 A181902 * A278707 A294631 A059454

Adjacent sequences:  A295628 A295629 A295630 * A295632 A295633 A295634

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Nov 24 2017

STATUS

approved

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Last modified May 24 06:53 EDT 2019. Contains 323529 sequences. (Running on oeis4.)