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A256228 Numbers k such that 4*10^k - 21 is prime. 0
1, 2, 4, 5, 7, 9, 10, 17, 21, 41, 51, 59, 61, 77, 79, 83, 97, 108, 427, 615, 869, 900, 966, 3150, 3239, 3932, 5218, 11941, 30558, 44697, 90334, 113874, 128343, 142810 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For k>1, numbers that begin with the digit 3 followed by k-2 occurrences of the digit 9 followed by the digits 79 are prime (see Example section).

a(36) > 2*10^5.

LINKS

Table of n, a(n) for n=1..34.

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

Makoto Kamada, Search for 39w79.

EXAMPLE

4 is in this sequence because 4*10^4 - 21 = 39979; is prime.

Initial terms and primes associated:

a(1) = 1, 19;

a(2) = 2, 379;

a(3) = 4, 39979;

a(4) = 5, 399979;

a(5) = 7, 39999979, etc.

MATHEMATICA

Select[Range[1, 100000], PrimeQ[4*10^# - 21] &]

PROG

(PARI) is(n)=ispseudoprime(4*10^n - 21) \\ Charles R Greathouse IV, Jun 13 2017

CROSSREFS

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

Sequence in context: A050731 A295579 A098794 * A114055 A169581 A184858

Adjacent sequences:  A256225 A256226 A256227 * A256229 A256230 A256231

KEYWORD

nonn,more

AUTHOR

Robert Price, Apr 17 2016

EXTENSIONS

a(32)-a(34) from Robert Price, Sep 10 2018

STATUS

approved

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Last modified August 13 04:54 EDT 2020. Contains 336442 sequences. (Running on oeis4.)