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A281646
Numbers k such that (25*10^k - 241)/9 is prime.
0
2, 4, 5, 8, 20, 35, 80, 131, 169, 262, 364, 484, 580, 692, 1687, 1834, 3476, 3950, 5963, 6329, 13339, 18617, 72703, 103360
OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 7 followed by the digits 51 is prime (see Example section).
a(25) > 2*10^5.
EXAMPLE
4 is in this sequence because (25*10^4 - 241)/9 = 27751 is prime.
Initial terms and associated primes:
a(1) = 2, 251;
a(2) = 4, 27751;
a(3) = 5, 277751;
a(4) = 8, 277777751;
a(5) = 20, 277777777777777777751; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(25*10^# - 241)/9] &]
PROG
(PARI) isok(k) = ispseudoprime((25*10^k-241)/9); \\ Altug Alkan, May 07 2018
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 25 2017
EXTENSIONS
a(24) from Robert Price, May 07 2018
STATUS
approved