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A293687
Numbers k such that (11*10^k - 137)/9 is prime.
0
2, 5, 21, 23, 47, 75, 107, 110, 188, 261, 617, 872, 965, 2735, 10145, 12572, 13364, 14570, 195350
OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 2 followed by the digits 07 is prime (see Example section).
a(20) > 2*10^5.
EXAMPLE
2 is in this sequence because (11*10^2 - 137)/9 = 107 is prime.
Initial terms and associated primes:
a(1) = 2, 107;
a(2) = 5, 122207;
a(3) = 21, 1222222222222222222207;
a(4) = 23, 122222222222222222222207;
a(5) = 47, 122222222222222222222222222222222222222222222207; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[(11*10^# - 137)/9] &]
PROG
(PARI) isok(k) = isprime((11*10^k - 137)/9); \\ Michel Marcus, Nov 15 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 15 2017
EXTENSIONS
a(19) from Robert Price, Jan 26 2018
STATUS
approved