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A276545 Numbers k such that (43*10^k - 421)/9 is prime. 0
2, 5, 7, 8, 11, 13, 25, 26, 61, 82, 131, 289, 377, 547, 845, 929, 1786, 5887, 6562, 10546, 28033, 33493, 150515 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For k>1, numbers such that the digit 4 followed by k-2 occurrences of the digit 7 followed by the digits 31 is prime (see Example section).

a(24) > 2*10^5.

LINKS

Table of n, a(n) for n=1..23.

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

Makoto Kamada, Search for 47w31.

EXAMPLE

4 is in this sequence because (43*10^4 - 421)/9 = 477731 is prime.

Initial terms and primes associated:

a(1) = 2, 431;

a(2) = 5, 477731;

a(3) = 7, 47777731;

a(4) = 8, 477777731;

a(5) = 11, 477777777731; etc.

MATHEMATICA

Select[Range[1, 100000], PrimeQ[(43*10^# - 421)/9] &]

CROSSREFS

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

Sequence in context: A051279 A288464 A111199 * A138671 A286693 A032724

Adjacent sequences:  A276542 A276543 A276544 * A276546 A276547 A276548

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Apr 09 2017

EXTENSIONS

a(23) from Robert Price, Jan 21 2019

STATUS

approved

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Last modified May 25 12:34 EDT 2022. Contains 354071 sequences. (Running on oeis4.)