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A278336
Numbers k such that (94*10^k - 7) / 3 is prime.
0
0, 1, 4, 5, 9, 16, 20, 41, 43, 63, 127, 159, 413, 1591, 1812, 2031, 2315, 2437, 4177, 4860, 5771, 7060, 7389, 9925, 34103, 115879
OFFSET
1,3
COMMENTS
For k > 0, numbers k such that the digits 31 followed by k-1 occurrences of the digit 3 followed by the digit 1 is prime (see Example section).
a(27) > 2*10^5.
EXAMPLE
4 is in this sequence because (94*10^4 - 7) / 3 = 313331 is prime.
Initial terms and associated primes:
a(1) = 0, 29;
a(2) = 1, 311;
a(3) = 4, 313331;
a(4) = 5, 3133331;
a(5) = 9, 31333333331; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(94*10^# - 7) / 3] &]
PROG
(PARI) is(n)=ispseudoprime((94*10^n - 7)/3) \\ Charles R Greathouse IV, Jun 13 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 18 2016
EXTENSIONS
a(26) from Robert Price, Mar 10 2020
STATUS
approved