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A210711 Semiprimes formed by concatenating n, n, and 1 for n = 1, 2, 3,....

%I A210711 #20 Sep 08 2022 08:46:01
%S A210711 111,221,551,771,11111,14141,15151,16161,19191,23231,24241,29291,
%T A210711 30301,34341,36361,37371,38381,39391,42421,44441,47471,50501,53531,
%U A210711 55551,56561,59591,62621,68681,70701,74741,75751,77771,79791,81811,83831,84841,87871,91911,95951,96961,1001001
%N A210711 Semiprimes formed by concatenating n, n, and 1 for n = 1, 2, 3,....
%C A210711 This is to A210511 as semiprimes A001358 are to primes A000040.
%H A210711 Robert Israel, <a href="/A210711/b210711.txt">Table of n, a(n) for n = 1..10000</a>
%e A210711 a(1) = 11 because 111 = 3 * 37.
%e A210711 a(2) = 221 because 221 = 13 * 17.
%e A210711 331 is not in the sequence, because it is a prime.
%e A210711 a(5) = 11111 because "11" concatenated with "11" concatenated with "1" = 11111 = 41 * 271.
%p A210711 read("transforms"):
%p A210711 for n from 1 to 100 do
%p A210711     L := [n,n,1] ;
%p A210711     p := digcatL(L) ;
%p A210711     if numtheory[bigomega](p) = 2 then
%p A210711         print(p) ;
%p A210711     end if;
%p A210711 end do: # _R. J. Mathar_, Jan 30 2013
%o A210711 (Magma)
%o A210711 IsSemiprime:=func<i|&+[d[2]: d in Factorization(i)] eq 2>;
%o A210711 [nn1: n in [1..100] | IsSemiprime(nn1) where nn1 is Seqint([1] cat Intseq(n) cat Intseq(n))]; // _Bruno Berselli_, Jan 30 2013
%Y A210711 Cf. A001358, A210511.
%K A210711 nonn,base,easy
%O A210711 1,1
%A A210711 _Jonathan Vos Post_, Jan 29 2013