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A290431
Numbers k such that (73*10^k + 413)/9 is prime.
0
1, 2, 4, 5, 7, 16, 31, 34, 44, 68, 145, 158, 227, 499, 643, 970, 1004, 1951, 2923, 3092, 28069, 48334, 76262
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 1 followed by the digits 57 is prime (see Example section).
a(24) > 2*10^5.
EXAMPLE
4 is in this sequence because (73*10^4 + 413)/9 = 81157 is prime.
Initial terms and associated primes:
a(1) = 1, 127;
a(2) = 2, 857;
a(3) = 4, 81157;
a(4) = 5, 811157;
a(5) = 7, 81111157; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(73*10^# + 413)/9] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 06 2017
STATUS
approved