|
|
A252656
|
|
Numbers n such that 3^n - n is a semiprime.
|
|
8
|
|
|
4, 6, 10, 25, 28, 32, 98, 124, 146, 164, 182, 190, 200, 220, 226, 230, 248, 280, 362, 376, 418, 446, 518, 544
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
1,1
|
|
COMMENTS
|
Are there odd members of the sequence other than 25? There are no others < 10000. An odd number m is in the sequence iff (3^m - m)/2 is prime. - Robert Israel, Jan 02 2015
No more odd terms after a(4) = 25 for m < 200000. a(25) >= 626. - Hugo Pfoertner, Aug 07 2019
|
|
LINKS
|
|
|
EXAMPLE
|
4 is in this sequence because 3^4 - 4 = 7*11 is semiprime.
10 is in this sequence because 3^10 - 10 = 43*1373 and these two factors are prime.
|
|
MAPLE
|
select(n -> numtheory:-bigomega(3^n - n) = 2, [$1..150]); # Robert Israel, Jan 02 2015
|
|
MATHEMATICA
|
Select[Range[150], PrimeOmega[3^# - #] == 2 &]
|
|
PROG
|
(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [m: m in [2..150] | IsSemiprime(s) where s is 3^m-m];
(PARI) n=1; while(n<100, s=3^n-n; c=0; forprime(p=1, 10^4, if(s%p, c++); if(s%p==0, s1=s/p; if(ispseudoprime(s1), print1(n, ", "); c=0; break); if(!ispseudoprime(s1), c=0; break))); if(!c, n++); if(c, if(bigomega(s)==2, print1(n, ", ")); n++)) \\ Derek Orr, Jan 02 2015
|
|
CROSSREFS
|
|
|
KEYWORD
|
nonn,more,hard
|
|
AUTHOR
|
|
|
EXTENSIONS
|
|
|
STATUS
|
approved
|
|
|
|