A252795 - OEIS

%I #22 May 02 2022 10:27:08

%S 3,7,37,43,49,51,57,73,127

%N Numbers m such that 10^m + m is a semiprime.

%C a(10) >= 217. - _Hugo Pfoertner_, Aug 06 2019

%C 574 and 601 are also terms in this sequence. - _Kevin P. Thompson_, May 02 2022

%H factordb.com, <a href="http://factordb.com/index.php?query=10%5E217%2B217">Status of 10^217+217</a>.

%e 3 is in this sequence because 10^3+3 = 17*59 is semiprime.

%e 7 is in this sequence because 10^7+7 = 941*10627 and these two factors are prime.

%t Select[Range[70], PrimeOmega[10^# + #]==2 &]

%o (Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [m: m in [1..70] | IsSemiprime(s) where s is 10^m+m];

%o (PARI) is(n)=bigomega(10^n + n)==2 \\ _Anders Hellström_, Aug 15 2015

%Y Cf. similar sequences listed in A252788.

%Y Cf. A089379, A252663.

%K nonn,more,hard

%O 1,1

%A _Vincenzo Librandi_, Dec 27 2014

%E a(8) from _Carl Schildkraut_, Aug 15 2015

%E Missing a(8) term added by _Kevin P. Thompson_, May 02 2022