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A281839
Numbers k such that 5*10^k + 21 is prime.
0
1, 2, 3, 4, 7, 10, 17, 22, 32, 33, 39, 70, 79, 97, 273, 304, 905, 2474, 3523, 10348, 15155, 22252, 22966, 70858, 82504, 90793
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 0 followed by the digits 21 is prime (see Example section).
a(27) > 2*10^5.
EXAMPLE
4 is in this sequence because 5*10^4 + 21 = 50021 is prime.
Initial terms and associated primes:
a(1) = 1, 71;
a(2) = 2, 521;
a(3) = 3, 5021;
a(4) = 4, 50021;
a(5) = 7, 50000021; etc.
MAPLE
select(k->isprime(5*10^k+21), [$1..1000]); # Muniru A Asiru, Jan 05 2019
MATHEMATICA
Select[Range[0, 100000], PrimeQ[5*10^# + 21] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 31 2017
STATUS
approved