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A295082
Numbers k such that (17*10^k - 521)/9 is prime.
0
2, 3, 5, 8, 9, 14, 20, 21, 29, 41, 113, 677, 3011, 3015, 3083, 3140, 4473, 4947, 138812
OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 8 followed by the digits 31 is prime (see Example section).
a(20) > 2*10^5.
EXAMPLE
2 is in this sequence because (17*10^2 - 521)/9 = 131 is prime.
Initial terms and associated primes:
a(1) = 2, 131;
a(2) = 3, 1831;
a(3) = 5, 188831;
a(4) = 8, 188888831;
a(5) = 9, 1888888831; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[(17*10^# - 521)/9] &]
PROG
(PARI) isok(k) = isprime((17*10^k - 521)/9); \\ Michel Marcus, Nov 14 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 13 2017
EXTENSIONS
a(19) from Robert Price, Apr 24 2018
STATUS
approved