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A290150
Numbers k such that (316*10^k - 1)/9 is prime.
0
2, 3, 8, 9, 11, 14, 17, 23, 29, 38, 50, 129, 164, 578, 1592, 2951, 4505, 16826, 17438, 22748
OFFSET
1,1
COMMENTS
Numbers k such that the digits 35 followed by k occurrences of the digit 1 is prime (see Example section).
a(21) > 2*10^5.
EXAMPLE
3 is in this sequence because (316*10^3 - 1)/9 = 35111 is prime.
Initial terms and associated primes:
a(1) = 2, 3511;
a(2) = 3, 35111;
a(3) = 8, 3511111111;
a(4) = 9, 35111111111;
a(5) = 11, 3511111111111; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(316*10^# - 1)/9] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jul 21 2017
STATUS
approved