OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 9 followed by k-2 occurrences of the digit 4 followed by the digits 97 is prime (see Example section).
a(28) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 94w97.
EXAMPLE
3 is in this sequence because (85*10^3 + 473)/9 = 9497 is prime.
Initial terms and associated primes:
a(1) = 2, 997;
a(2) = 3, 9497;
a(3) = 5, 944497;
a(4) = 6, 9444497;
a(5) = 8, 944444497; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[ (85*10^# + 473)/9 ] &]
PROG
(PARI) isok(n) = isprime((85*10^n + 473)/9); \\ Indranil Ghosh, Mar 09 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Mar 09 2017
EXTENSIONS
a(25)-a(27) from Robert Price, Jan 06 2020
STATUS
approved