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A295387 Numbers k such that (2*10^k - 197)/3 is prime. 0
3, 5, 17, 20, 21, 107, 176, 207, 2115, 2156, 2571, 4082, 5291, 7148, 7525, 16857, 23245, 23347, 31307, 35107, 54527 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For k > 1, numbers such that k-2 occurrences of the digit 6 followed by the digits 01 is prime (see Example section).

a(22) > 2*10^5.

LINKS

Table of n, a(n) for n=1..21.

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

Makoto Kamada, Search for 6w01

EXAMPLE

3 is in this sequence because (2*10^3 - 197)/3 = 601 is prime.

Initial terms and primes associated:

a(1) = 3, 601;

a(2) = 5, 66601;

a(3) = 17, 66666666666666601;

a(4) = 20, 66666666666666666601;

a(5) = 21, 666666666666666666601; etc.

MATHEMATICA

Select[Range[2, 100000], PrimeQ[(2*10^# - 197)/3] &]

PROG

(PARI) isok(k) = isprime((2*10^k - 197)/3); \\ Michel Marcus, Nov 22 2017

CROSSREFS

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

Sequence in context: A287638 A119768 A020592 * A263258 A326653 A218624

Adjacent sequences:  A295384 A295385 A295386 * A295388 A295389 A295390

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Nov 21 2017

STATUS

approved

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Last modified July 16 09:13 EDT 2020. Contains 335784 sequences. (Running on oeis4.)