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A295327
Numbers k such that 3*10^k - 13 is prime.
0
1, 13, 16, 19, 23, 25, 29, 30, 34, 53, 61, 89, 111, 191, 376, 1918, 2889, 12304, 17939, 18053, 31740, 56575, 65899
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 9 followed by the digits 87 is prime (see Example section).
a(24) > 2*10^5.
EXAMPLE
1 is in this sequence because 3*10^1 - 13 = 17 is prime.
Initial terms and associated primes:
a(1) = 1, 17;
a(2) = 13, 29999999999987;
a(3) = 16, 29999999999999987;
a(4) = 19, 29999999999999999987;
a(5) = 23, 299999999999999999999987; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[3*10^# - 13] &]
PROG
(PARI) isok(k) = isprime(3*10^k - 13); \\ Michel Marcus, Nov 20 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 19 2017
STATUS
approved