A252661 Numbers m such that 8^m - m is a semiprime.

2, 7, 9, 111, 129, 133, 361

Offset

1,1

Comments

a(8) >= 369. - Hugo Pfoertner, Aug 03 2019

Links

Table of n, a(n) for n=1..7.

factordb.com, Status of 8^369-369.

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

Adjacent sequences: A252658 A252659 A252660 * A252662 A252663 A252664

Keyword

nonn,more,hard

Author

Vincenzo Librandi, Dec 21 2014

Extensions

a(4)-a(7) from Hugo Pfoertner, Aug 03 2019

Status

approved