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A281991 Numbers k such that (598*10^k - 31)/9 is prime. 0
1, 4, 5, 7, 16, 17, 19, 28, 59, 68, 128, 307, 343, 368, 421, 448, 469, 691, 1949, 2506, 4868, 6914, 30025, 30955, 35522, 43388, 45716, 49999, 76309, 80149 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For k>1, numbers such that the digits 64 followed by k-1 occurrences of the digit 4 followed by the digit 1 is prime (see Example section).

a(31) > 10^5.

LINKS

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

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

Makoto Kamada, Search for 664w1.

EXAMPLE

4 is in this sequence because (598*10^4 - 31)/9 = 664441 is prime.

Initial terms and primes associated:

a(1) = 1, 661;

a(2) = 4, 664441;

a(3) = 5, 6644441;

a(4) = 7, 664444441;

a(5) = 16, 664444444444444441; etc.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[(598*10^# - 31)/9] &]

PROG

(PARI) isok(k) = isprime((598*10^k - 31)/9); \\ Michel Marcus, Feb 04 2017

CROSSREFS

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

Sequence in context: A250182 A072215 A106170 * A216537 A085195 A107289

Adjacent sequences:  A281988 A281989 A281990 * A281992 A281993 A281994

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Feb 04 2017

STATUS

approved

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Last modified December 11 21:00 EST 2019. Contains 329937 sequences. (Running on oeis4.)