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A273907 Numbers k such that 2*10^k - 87 is prime. 0
2, 3, 4, 9, 10, 13, 15, 24, 26, 38, 39, 42, 433, 489, 495, 513, 597, 829, 2019, 2738, 3096, 3691, 5437, 7537, 8536, 34125, 40105, 41790, 52713, 104811, 173809, 175860 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For k>1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 9 followed by the digits 13 is prime (see Example section).

a(33) > 2*10^5.

LINKS

Table of n, a(n) for n=1..32.

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

Makoto Kamada, Search for 19w13.

EXAMPLE

3 is in this sequence because 2*10^3 - 87 = 1913 is prime.

Initial terms and primes associated:

a(1) = 2, 113;

a(2) = 3, 1913;

a(3) = 4, 19913;

a(4) = 9, 1999999913;

a(5) = 10, 19999999913, etc.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[2*10^# - 87] &]

PROG

(PARI) is(n)=ispseudoprime(2*10^n - 87) \\ Charles R Greathouse IV, Jun 08 2016

CROSSREFS

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

Sequence in context: A073338 A200260 A190119 * A066105 A083180 A098551

Adjacent sequences:  A273904 A273905 A273906 * A273908 A273909 A273910

KEYWORD

nonn,more

AUTHOR

Robert Price, Jun 03 2016

EXTENSIONS

a(30)-a(32) from Robert Price, Apr 13 2018

STATUS

approved

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Last modified September 16 06:18 EDT 2021. Contains 347469 sequences. (Running on oeis4.)