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A281144 Numbers k such that (302*10^k + 13)/9 is prime. 0
1, 4, 7, 9, 16, 33, 55, 63, 75, 195, 412, 766, 1083, 1657, 2254, 4972, 10053, 10605, 13413, 18085, 24595, 25161, 36441, 80638, 178369 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For k>0, numbers such that the digits 33 followed by k-1 occurrences of the digit 5 followed by the digit 7 is prime (see Example section).

a(26) > 2*10^5.

LINKS

Table of n, a(n) for n=1..25.

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

Makoto Kamada, Search for 335w7.

EXAMPLE

4 is in this sequence because (302*10^4 + 13)/9 = 335557 is prime.

Initial terms and primes associated:

a(1) = 1, 337;

a(2) = 4, 335557;

a(3) = 7, 335555557;

a(4) = 9, 33555555557;

a(5) = 16, 335555555555555557; etc.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[(302*10^# + 13)/9] &]

CROSSREFS

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

Sequence in context: A271542 A010424 A044903 * A121865 A103073 A166742

Adjacent sequences:  A281141 A281142 A281143 * A281145 A281146 A281147

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Jan 15 2017

EXTENSIONS

a(25) from Robert Price, Feb 08 2020

STATUS

approved

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Last modified January 25 23:56 EST 2022. Contains 350572 sequences. (Running on oeis4.)