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A280431 Numbers k such that (2*10^k - 41)/3 is prime. 0
2, 3, 4, 5, 8, 9, 27, 32, 39, 44, 101, 140, 268, 386, 695, 741, 1155, 1564, 1611, 4076, 6083, 11672, 26685, 50603, 134720 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For n>1, numbers such that n-2 occurrences of the digit 6 followed by the digits 53 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 6w53.

EXAMPLE

4 is in this sequence because (2*10^4 - 41) / 3 = 6653 is prime.

Initial terms and primes associated:

a(1) = 2, 53;

a(2) = 3, 653;

a(3) = 4, 6653;

a(4) = 5, 66653;

a(5) = 8, 66666653; etc.

MATHEMATICA

Select[Range[2, 100000], PrimeQ[(2*10^# - 41) / 3] &]

PROG

(PARI) is(n)=ispseudoprime((2*10^n-41)/3) \\ Charles R Greathouse IV, Jun 13 2017

CROSSREFS

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

Sequence in context: A152606 A057911 A085266 * A056804 A305399 A101547

Adjacent sequences:  A280428 A280429 A280430 * A280432 A280433 A280434

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Jan 02 2017

EXTENSIONS

a(25) from Robert Price, Feb 14 2018

STATUS

approved

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Last modified August 4 11:13 EDT 2021. Contains 346447 sequences. (Running on oeis4.)