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A294680
Numbers k such that (85*10^k + 293)/9 is prime.
0
1, 2, 4, 10, 23, 41, 47, 61, 137, 278, 388, 523, 992, 3716, 4438, 5185, 7553, 9970, 17962, 18568, 36796, 112919, 114737, 116842
OFFSET
1,2
COMMENTS
For k>1, numbers k such that the digit 9 followed by k-2 occurrences of the digit 4 followed by the digits 77 is prime (see Example section).
a(25) > 2*10^5.
EXAMPLE
2 is in this sequence because (85*10^2 + 293)/9 = 977 is prime.
Initial terms and associated primes:
a(1) = 1, 127;
a(2) = 2, 977;
a(3) = 4, 94477;
a(4) = 10, 94444444477;
a(5) = 23, 944444444444444444444477; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(85*10^# + 293)/9] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 06 2017
EXTENSIONS
a(22)-a(24) from Robert Price, Dec 13 2019
STATUS
approved