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URL: https://oeis.org/A276545

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A276545
Numbers k such that (43*10^k - 421)/9 is prime.
0
2, 5, 7, 8, 11, 13, 25, 26, 61, 82, 131, 289, 377, 547, 845, 929, 1786, 5887, 6562, 10546, 28033, 33493, 150515, 205183
OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 7 followed by the digits 31 is prime (see Example section).
a(25) > 3*10^5.
EXAMPLE
4 is in this sequence because (43*10^4 - 421)/9 = 477731 is prime.
Initial terms and associated primes:
a(1) = 2, 431;
a(2) = 5, 477731;
a(3) = 7, 47777731;
a(4) = 8, 477777731;
a(5) = 11, 477777777731; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(43*10^# - 421)/9] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Apr 09 2017
EXTENSIONS
a(23) from Robert Price, Jan 21 2019
a(24) from Robert Price, Oct 25 2023
STATUS
approved