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A295402
Numbers k such that (28*10^k + 701)/9 is prime.
0
1, 2, 4, 5, 14, 17, 19, 29, 40, 56, 62, 82, 104, 121, 181, 286, 1307, 2197, 5764, 37120, 91715
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 1 followed by the digits 89 is prime (see Example section).
a(22) > 2*10^5.
EXAMPLE
2 is in this sequence because (28*10^2 + 701)/9 = 389 is prime.
Initial terms and associated primes:
a(1) = 1, 109;
a(2) = 2, 389;
a(3) = 4, 31189;
a(4) = 5, 311189;
a(5) = 14, 311111111111189; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(28*10^# + 701)/9] &]
PROG
(PARI) isok(k) = isprime((28*10^k + 701)/9); \\ Michel Marcus, Nov 22 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 21 2017
STATUS
approved