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A281989 Numbers k such that 7*10^k + 61 is prime. 0
1, 2, 4, 6, 9, 14, 18, 20, 23, 24, 30, 61, 150, 209, 213, 226, 231, 364, 455, 3990, 15267, 24289, 28270, 94938, 155496 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For k>1, numbers such that the digit 7 followed by k-2 occurrences of the digit 0 followed by the digits 61 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 70w61.

EXAMPLE

4 is in this sequence because 7*10^4 + 61 = 70061 is prime.

Initial terms and primes associated:

a(1) = 1, 131;

a(2) = 2, 761;

a(3) = 4, 70061;

a(4) = 6, 7000061;

a(5) = 9, 7000000061; etc.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[7*10^# + 61] &]

PROG

(PARI) isok(k) = isprime(7*10^k + 61); \\ Michel Marcus, Feb 04 2017

CROSSREFS

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

Sequence in context: A205883 A253108 A198201 * A328423 A070211 A113753

Adjacent sequences:  A281986 A281987 A281988 * A281990 A281991 A281992

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Feb 04 2017

EXTENSIONS

a(25) from Robert Price, Aug 20 2019

STATUS

approved

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Last modified September 28 17:45 EDT 2022. Contains 357080 sequences. (Running on oeis4.)