A252790 - OEIS

%I #20 Sep 08 2022 08:46:10

%S 1,4,8,17,144,154,298,572,732

%N Numbers m such that 5^m + m is a semiprime.

%C a(8) > 413. - _Carl Schildkraut_, Aug 14 2015

%C a(10) >= 766. - _Kevin P. Thompson_, May 01 2022

%H FactorDB, <a href="http://factordb.com/index.php?id=1100000000212735410">Status of 5^766+766</a>

%e 1 is in this sequence because 5^1+1 = 2*3 is semiprime.

%e 8 is in this sequence because 5^8+8 = 3*130211 and these two factors are prime.

%t Select[Range[413], PrimeOmega[5^# + #]==2 &]

%o (Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [m: m in [1..110] | IsSemiprime(s) where s is 5^m+m];

%o (PARI) main(m)=select(m->bigomega(5^m + m)==2,vector(m,i,i)); \\ _Anders Hellström_, Aug 14 2015

%Y Cf. similar sequences listed in A252788.

%Y Cf. A252658.

%K nonn,more

%O 1,2

%A _Vincenzo Librandi_, Dec 24 2014

%E a(5)-a(7) from _Carl Schildkraut_, Aug 14 2015

%E a(8)-a(9) from _Kevin P. Thompson_, May 01 2022