OFFSET
1,1
COMMENTS
549, 721, and 755 are in the sequence, but not necessarily the next three terms. The other possibilities for a(9) are 483, 503, and 543. - Robert Israel, Feb 10 2019
LINKS
factordb.com, Status of 4^483-483.
EXAMPLE
2 is in this sequence because 4^2-2 = 2*7 is semiprime.
17 is in this sequence because 4^17-17 = 6971*2464477 and these two factors are prime.
MATHEMATICA
Select[Range[120], PrimeOmega[4^# - #]==2 &]
PROG
(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [m: m in [2..120] | IsSemiprime(s) where s is 4^m-m];
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Vincenzo Librandi, Dec 20 2014
EXTENSIONS
a(8)-a(9) from Luke March, Jul 08 2015
STATUS
approved