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A294941
Numbers k such that (14*10^k + 211)/9 is prime.
0
2, 3, 5, 6, 8, 9, 18, 41, 209, 815, 2573, 3590, 4439, 4568, 6720, 16491, 20069, 22406, 36756, 43076, 53465, 58550, 70896, 74858, 184950
OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 5 followed by the digits 79 is prime (see Example section).
a(26) > 2*10^5.
EXAMPLE
2 is in this sequence because (14*10^2 + 211)/9 = 179 s prime.
Initial terms and associated primes:
a(1) = 2, 179;
a(2) = 3, 1579;
a(3) = 5, 155579;
a(4) = 6, 1555579;
a(5) = 8, 155555579; etc.
MATHEMATICA
Select[Range[0 100000], PrimeQ[(14*10^# + 211)/9] &]
PROG
(PARI) isok(k) = isprime((14*10^k + 211)/9); \\ Michel Marcus, Nov 12 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 11 2017
EXTENSIONS
a(25) from Robert Price, Apr 21 2018
STATUS
approved