OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 6 followed by the digits 41 is prime (see Example section).
a(31) > 3*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 26w41.
EXAMPLE
4 is in this sequence because (8*10^4 - 77)/3 = 26641 is prime.
Initial terms and associated primes:
a(1) = 2, 241;
a(2) = 4, 26641;
a(3) = 5, 266641;
a(4) = 6, 2666641;
a(5) = 15, 2666666666666641, etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(8*10^# - 77)/3] &]
PROG
(PARI) lista(nn) = {for(n=1, nn, if(ispseudoprime((8*10^n - 77)/3), print1(n, ", "))); } \\ Altug Alkan, May 11 2016
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert Price, May 10 2016
EXTENSIONS
a(29) from Robert Price, Jul 07 2018
a(30) from Robert Price, Jul 02 2023
STATUS
approved