OFFSET
1,1
COMMENTS
a(12) >= 250. - Hugo Pfoertner, Aug 03 2019
LINKS
factordb.com, Status of 9^250-250.
EXAMPLE
4 is in this sequence because 9^4-4 = 79*83 and these two factors are prime.
14 is in this sequence because 9^14-14 = 163*140348419969 and these two factors are prime.
MATHEMATICA
Select[Range[80], PrimeOmega[9^# - #]==2 &]
PROG
(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [m: m in [1..80] | IsSemiprime(s) where s is 9^m-m];
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Vincenzo Librandi, Dec 22 2014
EXTENSIONS
a(11) from Hugo Pfoertner, Aug 03 2019
STATUS
approved