A252657 Numbers m such that 4^m - m is a semiprime.

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

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

Table of n, a(n) for n=1..9.

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

Cf. A024037 (4^n - n).

Cf. similar sequences listed in A252656.

Sequence in context: A141176 A118839 A091735 * A296053 A234647 A106949

Adjacent sequences: A252654 A252655 A252656 * A252658 A252659 A252660

Keyword

nonn,more

Author

Vincenzo Librandi, Dec 20 2014

Extensions

a(8)-a(9) from Luke March, Jul 08 2015

Status

approved