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A295387
Numbers k such that (2*10^k - 197)/3 is prime.
0
3, 5, 17, 20, 21, 107, 176, 207, 2115, 2156, 2571, 4082, 5291, 7148, 7525, 16857, 23245, 23347, 31307, 35107, 54527
OFFSET
1,1
COMMENTS
For k > 1, numbers k such that k-2 occurrences of the digit 6 followed by the digits 01 is prime (see Example section).
a(22) > 2*10^5.
EXAMPLE
3 is in this sequence because (2*10^3 - 197)/3 = 601 is prime.
Initial terms and associated primes:
a(1) = 3, 601;
a(2) = 5, 66601;
a(3) = 17, 66666666666666601;
a(4) = 20, 66666666666666666601;
a(5) = 21, 666666666666666666601; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[(2*10^# - 197)/3] &]
PROG
(PARI) isok(k) = isprime((2*10^k - 197)/3); \\ Michel Marcus, Nov 22 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 21 2017
STATUS
approved