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A282256
Numbers k such that (142*10^k - 7)/9 is prime.
0
1, 8, 16, 23, 32, 86, 202, 214, 332, 443, 760, 1066, 1270, 2210, 4189, 5759, 6886, 9286, 21205, 22139, 24784, 35021, 63734, 112352
OFFSET
1,2
COMMENTS
Numbers k such that the digits 15 followed by k occurrences of the digit 7 is prime (see Example section).
a(25) > 2*10^5.
EXAMPLE
8 is in this sequence because (142*10^8 - 7)/9 = 1577777777 is prime.
Initial terms and associated primes:
a(1) = 1, 157;
a(2) = 8, 1577777777;
a(3) = 16, 157777777777777777;
a(4) = 23, 1577777777777777777777777;
a(5) = 32, 1577777777777777777777777777777777; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(142*10^# - 7)/9] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 12 2017
EXTENSIONS
a(24) from Robert Price, Dec 31 2019
STATUS
approved