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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
OFFSET
1,1
COMMENTS
a(12) >= 250. - Hugo Pfoertner, Aug 03 2019
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
KEYWORD
nonn,more,hard
AUTHOR
Vincenzo Librandi, Dec 22 2014
EXTENSIONS
a(11) from Hugo Pfoertner, Aug 03 2019
STATUS
approved