OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 1 followed by the digits 43 is prime (see Example section).
a(32) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 71w43.
EXAMPLE
5 is in this sequence because (64*10^5 + 287)/9 = 711143 is prime.
Initial terms and associated primes:
a(1) = 1, 103;
a(2) = 2, 743;
a(3) = 4, 71143;
a(4) = 5, 711143;
a(5) = 7, 71111143, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(64*10^#n + 287)/9] &]
PROG
(PARI) lista(nn) = for(n=1, nn, if(ispseudoprime((64*10^n + 287)/9), print1(n, ", "))); \\ Altug Alkan, Apr 22 2016
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert Price, Apr 22 2016
EXTENSIONS
a(31) from Robert Price, Apr 13 2019
STATUS
approved