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A252657
Numbers m such that 4^m - m is a semiprime.
2
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
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
KEYWORD
nonn,more
AUTHOR
Vincenzo Librandi, Dec 20 2014
EXTENSIONS
a(8)-a(9) from Luke March, Jul 08 2015
STATUS
approved