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%I #17 Sep 08 2022 08:46:08
%S 1,2,4,16,21,40,76,113,153,288
%N Numbers n such that n*5^n+1 is semiprime.
%C The semiprimes of this form are 6, 51, 2501, 2441406250001, 10013580322265626, 363797880709171295166015625001, ...
%C a(11) >= 1006. - _Hugo Pfoertner_, Aug 05 2019
%H factordb.com, <a href="http://factordb.com/index.php?query=5%5E1006*1006%2B1">Status of 1006*5^1006+1</a>.
%t Select[Range[90], PrimeOmega[# 5^# + 1] == 2 &]
%o (Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [1..90] | IsSemiprime(s) where s is n*5^n+1];
%Y Cf. similar sequences listed in A242203.
%Y Cf. A050916.
%K nonn,more,hard
%O 1,2
%A _Vincenzo Librandi_, May 10 2014
%E a(8)-a(10) from _Luke March_, Aug 13 2015
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