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A282383
Numbers k such that (14*10^k - 221)/9 is prime.
0
2, 3, 15, 17, 21, 38, 93, 323, 410, 413, 444, 884, 1836, 2192, 3527, 4700, 6260, 6944, 8229, 10320, 27480, 31425, 32937, 108545, 155024, 182177, 196965
OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 5 followed by the digits 31 is prime (see Example section).
a(28) > 2*10^5.
EXAMPLE
3 is in this sequence because (14*10^3 - 221)/9 = 1531 is prime.
Initial terms and associated primes:
a(1) = 2, 131;
a(2) = 3, 1531;
a(3) = 15, 1555555555555531;
a(4) = 17, 155555555555555531;
a(5) = 21, 1555555555555555555531; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[(14*10^# - 221)/9] &]
PROG
(PARI) isok(k) = ispseudoprime((14*10^k - 221)/9); \\ Altug Alkan, Apr 12 2018
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 13 2017
EXTENSIONS
a(24)-a(27) from Robert Price, Apr 12 2018
STATUS
approved