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A294376
Numbers k such that 55*10^k + 7 is prime.
0
1, 2, 4, 7, 16, 33, 35, 51, 371, 1539, 1564, 2897, 3431, 4448, 6593, 7066, 7345, 11770, 20026, 22114, 23945, 46946, 60772
OFFSET
1,2
COMMENTS
For k > 0, numbers k such that the digits 55 followed by k-1 occurrences of the digit 0 followed by the digit 7 is prime (see Example section).
a(24) > 3*10^5.
EXAMPLE
2 is in this sequence because 55*10^2 + 7 = 5507 is prime.
Initial terms and associated primes:
a(1) = 1, 557;
a(2) = 2, 5507;
a(3) = 4, 550007;
a(4) = 7, 550000007;
a(5) = 16, 550000000000000007; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[55*10^# + 7] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 29 2017
STATUS
approved