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A293029
Numbers k such that (7*10^k - 439)/9 is prime.
0
2, 20, 34, 35, 65, 73, 89, 94, 103, 157, 208, 217, 430, 508, 1220, 4729, 5218, 6350, 11810, 29893, 37753, 89539, 165350
OFFSET
1,1
COMMENTS
For k > 1, numbers k such that k-2 occurrences of the digit 7 followed by the digits 29 is prime (see Example section).
a(24) > 2*10^5.
EXAMPLE
2 is in this sequence because (7*10^2 - 439)/9 = 29 is prime.
Initial terms and associated primes:
a(1) = 2, 29;
a(2) = 20, 77777777777777777729;
a(3) = 34, 7777777777777777777777777777777729;
a(4) = 35, 77777777777777777777777777777777729;
a(5) = 65, 77777777777777777777777777777777777777777777777777777777777777729; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(4*10^# + 137)/3] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Sep 28 2017
EXTENSIONS
a(23) from Robert Price, Feb 14 2018
STATUS
approved