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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(27) > 3*10^5.
LINKS
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
Sequence in context: A205883 A253108 A198201 * A328423 A070211 A113753
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 February 27 10:59 EST 2024. Contains 370378 sequences. (Running on oeis4.)