Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%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