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A293824
Numbers k such that 28*10^k + 1 is prime.
0
0, 1, 2, 3, 4, 5, 16, 19, 22, 37, 58, 547, 570, 817, 1068, 1909, 12809, 14470, 18209, 33176, 33412, 42176, 49798, 89401
OFFSET
1,3
COMMENTS
For k > 0, numbers k such that the digits 28 followed by k-1 occurrences of the digit 0 followed by the digit 1 is prime (see Example section).
a(25) > 10^5.
EXAMPLE
2 is in this sequence because 28*10^2 + 1 = 2801 is prime.
Initial terms and associated primes:
a(1) = 0, 29;
a(2) = 1, 281;
a(3) = 2, 2801;
a(4) = 3, 280001;
a(5) = 4, 2800001; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[28*10^# + 1] &]
PROG
(PARI) lista(nn) = for(n = 0, nn, if(isprime(28*10^n+1), print1(n, ", "))) \\ Iain Fox, Oct 18 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 16 2017
STATUS
approved