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A274911
Numbers k such that 7*10^k + 87 is prime.
0
1, 2, 5, 6, 18, 23, 59, 86, 115, 119, 251, 365, 370, 447, 1672, 3076, 3973, 5611, 7687, 8824, 13026, 17141, 17971, 23346, 29138, 94373, 94563, 142189, 156956, 255167, 266731
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 0 followed by the digits 87 is prime (see Example section).
a(32) > 3*10^5.
EXAMPLE
5 is in this sequence because 7*10^5 + 87 = 700087 is prime.
Initial terms and associated primes:
a(1) = 1, 157;
a(2) = 2, 787;
a(3) = 5, 700087;
a(4) = 6, 7000087;
a(5) = 18, 7000000000000000087, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[7*10^# + 87] &]
PROG
(PARI) is(n)=ispseudoprime(7*10^n + 87) \\ Charles R Greathouse IV, Jun 13 2017
KEYWORD
nonn,more
AUTHOR
Robert Price, Nov 11 2016
EXTENSIONS
a(28)-a(29) from Robert Price, Jul 27 2019
a(30)-a(31) from Robert Price, May 31 2023
STATUS
approved