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A294483
Numbers k such that (56*10^k + 403)/9 is prime.
0
1, 7, 8, 11, 35, 37, 49, 127, 149, 188, 1214, 3020, 5666, 6254, 7874, 8281, 11095, 13831, 20785, 20854, 31249, 40031, 40450, 63100
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 6 followed by k - 2 occurrences of the digit 2 followed by the digits 67 is prime (see Example section).
a(25) > 2*10^5.
EXAMPLE
1 is in this sequence because (56*10^1 + 403)/9 = 107 is prime.
Initial terms and associated primes:
a(1) = 1, 107;
a(2) = 7, 62222267;
a(3) = 8, 622222267;
a(4) = 11, 622222222267;
a(5) = 35, 622222222222222222222222222222222267; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(56 * 10^# + 403)/9] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 31 2017
STATUS
approved