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A252661 Numbers m such that 8^m - m is a semiprime. 2
2, 7, 9, 111, 129, 133, 361 (list; graph; refs; listen; history; text; internal format)
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 * A041453 A042157 A012937

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

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Last modified May 18 04:22 EDT 2021. Contains 343994 sequences. (Running on oeis4.)