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A295323
Numbers k such that 2*10^k - 51 is prime.
0
2, 3, 4, 8, 11, 13, 17, 28, 56, 105, 231, 339, 643, 922, 1219, 1880, 2209, 4238, 4987, 14770, 56194, 67043, 96867, 124751
OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 9 followed by the digits 49 is prime (see Example section).
a(25) > 2*10^5.
EXAMPLE
2 is in this sequence because 2*10^2 - 51 = 149 is prime.
Initial terms and associated primes:
a(1) = 2, 149;
a(2) = 3, 1949;
a(3) = 4, 19949;
a(4) = 8, 199999949;
a(5) = 11, 199999999949; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[2*10^# - 51] &]
PROG
(PARI) isok(k) = isprime(2*10^k - 51); \\ Michel Marcus, Nov 20 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 19 2017
EXTENSIONS
a(24) from Robert Price, Apr 13 2018
STATUS
approved