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A242271 Numbers n such that n*8^n+1 is semiprime. 1

%I #16 Sep 08 2022 08:46:08

%S 1,2,3,9,24,32,35,51,75,234,243,392,417,472

%N Numbers n such that n*8^n+1 is semiprime.

%C The semiprimes of this form are: 9, 129, 1537, 1207959553, 113336795588871485128705, 2535301200456458802993406410753, ...

%C a(15) >= 483. - _Hugo Pfoertner_, Aug 05 2019

%H factordb.com, <a href="http://factordb.com/index.php?query=8%5E483*483%2B1">Status of 483*8^483+1</a>.

%t Select[Range[80], PrimeOmega[# 8^# + 1] == 2 &]

%o (Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [1..80] | IsSemiprime(s) where s is n*8^n+1];

%Y Cf. similar sequences listed in A242203.

%Y Cf. A064746, A242178.

%K nonn,more,hard

%O 1,2

%A _Vincenzo Librandi_, May 10 2014

%E a(10)-a(14) from _Hugo Pfoertner_ and _Daniel Suteu_, Aug 05 2019

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Last modified March 28 18:04 EDT 2024. Contains 371254 sequences. (Running on oeis4.)