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A276047 Numbers k such that 4*10^k + 21 is prime. 0
1, 2, 3, 7, 15, 22, 30, 35, 44, 73, 89, 91, 224, 533, 821, 1037, 1338, 1458, 1777, 2046, 2257, 2877, 3047, 3407, 13398, 42766, 55906, 61625, 66653, 123113 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For k>1, numbers such that the digit 4 followed by k-2 occurrences of the digit 0 followed by the digits 21 is prime (see Example section).

a(31) > 2*10^5.

LINKS

Table of n, a(n) for n=1..30.

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

Makoto Kamada, Search for 40w21.

EXAMPLE

3 is in this sequence because 4*10^3 + 21 = 4021 is prime.

Initial terms and primes associated:

a(1) = 1, 61;

a(2) = 2, 421;

a(3) = 3, 4021;

a(4) = 7, 40000021;

a(5) = 15, 4000000000000021, etc.

MATHEMATICA

Select[Range[0, 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: A077161 A068080 A276600 * A209658 A098763 A001276

Adjacent sequences:  A276044 A276045 A276046 * A276048 A276049 A276050

KEYWORD

nonn,more

AUTHOR

Robert Price, Aug 17 2016

EXTENSIONS

a(30) from Robert Price, Dec 09 2018

STATUS

approved

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Last modified July 21 04:40 EDT 2019. Contains 325189 sequences. (Running on oeis4.)