A274986 -id:A274986 - cp's OEIS frontend

A294909 Numbers k such that (10^k + 47)/3 is prime.

1, 3, 5, 6, 9, 13, 120, 220, 568, 640, 666, 1205, 1600, 2479, 2482, 2939, 3077, 8649, 9520, 12407, 33421, 36162, 41190, 65340

Offset: 1

Vincenzo Librandi, Sep 26 2016

For k > 1, numbers k such that k-2 occurrences of the digit 3 followed by the digits 49 is prime (see Example section). - Robert Price, Nov 10 2017

a(25) > 3*10^5. - Robert Price, Oct 26 2023

			3 is in this sequence because (10^3 + 47)/3 = 349 is prime.
Initial terms and associated primes:
a(1) = 1, 19;
a(2) = 3, 349;
a(3) = 5, 33349;
a(4) = 6, 333349;
a(5) = 9, 333333349; etc.

Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 3w49.

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

See also similar sequences listed in A274986.

[n: n in [0..500] |IsPrime((10^n+47) div 3)];

Select[Range[0, 100000], PrimeQ[(10^# + 47)/3] &]

is(n) = ispseudoprime((10^n + 47) / 3); \\ Altug Alkan, Sep 26 2016

a(18)-a(24) from Robert Price, Nov 10 2017

cp's OEIS Frontend

A294909 Numbers k such that (10^k + 47)/3 is prime.

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