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A285570 Numbers k such that (47*10^k + 403)/9 is prime. 0
1, 4, 10, 12, 16, 28, 42, 108, 127, 664, 744, 781, 3036, 3184, 4753, 5695, 7350, 8592, 8694, 15939, 21822, 24466, 61821, 196281 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For k>1, numbers such that the digit 5 followed by k-2 occurrences of the digit 2 followed by the digits 67 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 52w67.

EXAMPLE

4 is in this sequence because (47*10^4 + 403)/9 = 52267 is prime.

Initial terms and primes associated:

a(1) = 1, 97;

a(2) = 4, 52267;

a(3) = 10, 52222222267;

a(4) = 12, 5222222222267;

a(5) = 16, 52222222222222267; etc.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[(47*10^# + 403)/9] &]

CROSSREFS

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

Sequence in context: A071565 A233741 A310340 * A109096 A079283 A190516

Adjacent sequences:  A285567 A285568 A285569 * A285571 A285572 A285573

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Apr 21 2017

EXTENSIONS

a(24) from Robert Price, Mar 07 2019

STATUS

approved

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Last modified February 24 20:33 EST 2020. Contains 332210 sequences. (Running on oeis4.)