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A295403
Numbers k such that (41*10^k - 383)/9 is prime.
0
1, 3, 5, 6, 21, 39, 114, 239, 246, 348, 906, 1115, 2801, 2958, 2982, 4275, 6581, 8736, 27051, 115523, 124335
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 5 followed by the digits 13 is prime (see Example section).
a(22) > 2*10^5.
EXAMPLE
3 is in this sequence because (41*10^3 - 383)/9 = 4513 is prime.
Initial terms and associated primes:
a(1) = 1, 3;
a(2) = 3, 4513;
a(3) = 5, 455513;
a(4) = 6, 4555513;
a(5) = 21, 4555555555555555555513; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(41*10^# - 383)/9] &]
PROG
(PARI) isok(k) = isprime((41*10^k - 383)/9); \\ Michel Marcus, Nov 22 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 21 2017
EXTENSIONS
a(20)-a(21) from Robert Price, Nov 16 2018
STATUS
approved