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A252662 Numbers m such that 9^m - m is a semiprime. 2
4, 8, 10, 14, 20, 26, 38, 44, 56, 80, 190 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

a(12) >= 250. - Hugo Pfoertner, Aug 03 2019

LINKS

Table of n, a(n) for n=1..11.

factordb.com, Status of 9^250-250.

EXAMPLE

4 is in this sequence because 9^4-4 = 79*83 and these two factors are prime.

14 is in this sequence because 9^14-14 = 163*140348419969 and these two factors are prime.

MATHEMATICA

Select[Range[80], PrimeOmega[9^# - #]==2 &]

PROG

(MAGMA) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [m: m in [1..80] | IsSemiprime(s) where s is 9^m-m];

CROSSREFS

Cf. similar sequences listed in A252656.

Sequence in context: A310995 A310996 A028873 * A310997 A310998 A153975

Adjacent sequences:  A252659 A252660 A252661 * A252663 A252664 A252665

KEYWORD

nonn,more,hard

AUTHOR

Vincenzo Librandi, Dec 22 2014

EXTENSIONS

a(11) from Hugo Pfoertner, Aug 03 2019

STATUS

approved

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Last modified September 19 15:09 EDT 2020. Contains 337178 sequences. (Running on oeis4.)