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A282667
Numbers k such that (7*10^k + 107)/3 is prime.
0
1, 2, 4, 8, 12, 15, 21, 31, 48, 151, 723, 879, 1811, 3444, 5104, 5512, 5695, 6723, 9082, 14944, 15184, 18512, 35141, 81985, 123563, 155203, 165134, 165239
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 3 followed by the digits 69 is prime (see Example section).
a(29) > 2*10^5.
EXAMPLE
4 is in this sequence because (7*10^4 + 107)/3 = 23369 is prime.
Initial terms and associated primes:
a(1) = 1, 59;
a(2) = 2, 269;
a(3) = 4, 23369;
a(4) = 8, 233333369;
a(5) = 12, 2333333333369; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(7*10^# + 107)/3] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 20 2017
EXTENSIONS
a(25)-a(28) from Robert Price, Feb 21 2018
STATUS
approved