A252657 - OEIS

%I #31 Sep 08 2022 08:46:10

%S 2,11,17,33,55,59,63,153,315

%N Numbers m such that 4^m - m is a semiprime.

%C 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

%H factordb.com, <a href="http://factordb.com/index.php?query=4%5E483-483">Status of 4^483-483</a>.

%e 2 is in this sequence because 4^2-2 = 2*7 is semiprime.

%e 17 is in this sequence because 4^17-17 = 6971*2464477 and these two factors are prime.

%t Select[Range[120], PrimeOmega[4^# - #]==2 &]

%o (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];

%Y Cf. A024037 (4^n - n).

%Y Cf. similar sequences listed in A252656.

%K nonn,more

%O 1,1

%A _Vincenzo Librandi_, Dec 20 2014

%E a(8)-a(9) from _Luke March_, Jul 08 2015