OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 8 followed by the digits 77 is prime (see Example section).
a(28) > 10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 28w77.
EXAMPLE
5 is in this sequence because (26*10^5 - 107)/9 = 288877 is prime.
Initial terms and associated primes:
a(1) = 1, 17;
a(2) = 2, 277;
a(3) = 5, 288877;
a(4) = 7, 28888877;
a(5) = 13, 28888888888877, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(26*10^# - 107)/9] &]
PROG
(PARI) lista(nn) = for(n=1, nn, if(ispseudoprime((26*10^n-107)/9), print1(n, ", "))); \\ Altug Alkan, Jul 22 2016
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert Price, Jul 21 2016
STATUS
approved