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A252661
Numbers m such that 8^m - m is a semiprime.
2
2, 7, 9, 111, 129, 133, 361
OFFSET
1,1
COMMENTS
a(8) >= 369. - Hugo Pfoertner, Aug 03 2019
EXAMPLE
2 is in this sequence because 8^2-2 = 2*31 is semiprime.
9 is in this sequence because 8^9-9 = 23*5835553 and these two factors are prime.
MATHEMATICA
Select[Range[70], PrimeOmega[8^# - #]==2 &]
PROG
(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [m: m in [1..70] | IsSemiprime(s) where s is 8^m-m];
CROSSREFS
Cf. similar sequences listed in A252656.
Sequence in context: A079942 A272412 A042561 * A362858 A041453 A042157
KEYWORD
nonn,more,hard
AUTHOR
Vincenzo Librandi, Dec 21 2014
EXTENSIONS
a(4)-a(7) from Hugo Pfoertner, Aug 03 2019
STATUS
approved