login
A288821
Numbers k such that 10^k - 401 is prime.
0
3, 6, 9, 11, 12, 25, 47, 166, 222, 234, 352, 407, 515, 646, 1161, 2475, 4329, 7191, 12894, 16007, 25819, 67579, 88457, 117135
OFFSET
1,1
COMMENTS
For k > 2, numbers k such that k-3 occurrences of the digit 9 followed by the digits 599 is prime (see Example section).
a(25) > 2*10^5.
EXAMPLE
6 is in this sequence because 10^6 - 401 = 999599 is prime.
Initial terms and associated primes:
a(1) = 3, 599;
a(2) = 6, 999599;
a(3) = 9, 999999599;
a(4) = 11, 99999999599;
a(5) = 12, 999999999599; etc.
MATHEMATICA
Select[Range[3, 100000], PrimeQ[10^# - 401] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jun 17 2017
EXTENSIONS
a(24) from Robert Price, Jan 19 2018
STATUS
approved