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%I #16 Sep 08 2022 08:46:08
%S 6,8,10,14,15,60,90,114,118,204,350,390
%N Numbers k such that k*7^k+1 is semiprime.
%C The semiprimes of this form are: 705895, 46118409, 2824752491, 9495123019887, 71213422649146, ...
%C a(13) >= 720. - _Kevin P. Thompson_, Apr 20 2022
%H FactorDB, <a href="http://factordb.com/index.php?id=1100000000045034824">Status of 720*7^720+1</a>
%t Select[Range[80], PrimeOmega[# 7^# + 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*7^n+1];
%o (PARI) is(k) = bigomega(k*7^k+1)==2;
%o for(k=0,120,if(k%4!=1,if(is(k),print1(k, ", ")))); \\ _Jinyuan Wang_, Apr 07 2019
%Y Cf. similar sequences listed in A242203.
%Y Cf. A050919, A242177.
%K nonn,more
%O 1,1
%A _Vincenzo Librandi_, May 10 2014
%E a(7)-a(9) from _Jinyuan Wang_, Apr 07 2019
%E a(10)-a(12) from _Kevin P. Thompson_, Apr 20 2022