login
A293280
Numbers k such that (28*10^k - 31)/3 is prime.
0
1, 3, 4, 13, 15, 24, 28, 48, 233, 558, 621, 1498, 1810, 2788, 3532, 3987, 4791, 7972, 12221, 13714, 15940, 41759, 46956, 47827, 51651, 102142
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 9 followed by k-2 occurrences of the digit 3 followed by the digits 23 is prime (see Example section).
a(27) > 2*10^5.
EXAMPLE
3 is in this sequence because (28*10^3 - 31)/3 = 9323 is prime.
Initial terms and associated primes:
a(1) = 1, 83;
a(2) = 3, 9323;
a(3) = 4, 93323;
a(4) = 13, 93333333333323;
a(5) = 15, 9333333333333323; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(28*10^# - 31)/3] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 04 2017
EXTENSIONS
a(26) from Robert Price, Dec 08 2019
STATUS
approved