login
A291710
Numbers k such that (418*10^k + 23)/9 is prime.
0
1, 3, 4, 7, 16, 33, 37, 226, 295, 313, 429, 607, 2113, 2317, 2721, 2887, 2907, 2953, 6058, 11971, 30865, 58923, 98199, 105343, 132441
OFFSET
1,2
COMMENTS
For k > 0, numbers k such that the digits 46 followed by k-1 occurrences of the digit 4 followed by the digit 7 is prime. See Example section.
a(26) > 2*10^5.
EXAMPLE
3 is in this sequence because (418*10^3 + 23)/9 = 46447 is prime.
Initial terms and associated primes:
a(1) = 1, 467;
a(2) = 3, 46447;
a(3) = 4, 464447;
a(4) = 7, 464444447;
a(5) = 16, 464444444444444447; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(418*10^# + 23)/9] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Aug 30 2017
EXTENSIONS
a(24)-a(25) from Robert Price, Feb 05 2020
STATUS
approved