cp's OEIS Frontend

A206447 Composite numbers n such that sigma(n) = sigma(d) has solution for some other composite number d.

%I A206447 #19 Dec 17 2017 22:05:00
%S A206447 14,15,16,20,24,25,26,28,30,33,35,38,39,40,42,44,46,48,51,54,55,56,58,
%T A206447 60,62,65,66,68,69,70,75,77,78,80,82,84,87,88,90,92,94,95,96,99,102,
%U A206447 104,105,108,110,112,114,115,116,118,119,120,122,123,124,125
%N A206447 Composite numbers n such that sigma(n) = sigma(d) has solution for some other composite number d.
%H A206447 Robert Israel, <a href="/A206447/b206447.txt">Table of n, a(n) for n = 1..10000</a>
%e A206447 Composite numbers 14 and 15 are in sequence because sigma(14) = sigma(15) = 24.
%p A206447 N:= 500:
%p A206447 Res:= {}: Q:= {}:
%p A206447 for n from 4 to N do
%p A206447   if isprime(n) then next fi;
%p A206447   s:= numtheory:-sigma(n);
%p A206447   if not assigned(V[s]) then
%p A206447      V[s]:= n;
%p A206447      if s > N then Q:= Q union {n} fi;
%p A206447   else
%p A206447      Res:= Res union {n,V[s]};
%p A206447      if s > N then Q:= Q minus {V[s]} fi;
%p A206447   fi
%p A206447 od:
%p A206447 convert(select(`<`,Res, min(Q)),list); # _Robert Israel_, Dec 17 2017
%t A206447 t2 = Table[If[PrimeQ[n], 0, DivisorSigma[1, n]], {n, 1000}]; Select[Range[132], ! PrimeQ[#] && Length[Position[t2, t2[[#]]]] > 1 &] (* _T. D. Noe_, Feb 27 2012 *)
%Y A206447 Cf. A206036, A158913, A066073.
%K A206447 nonn
%O A206447 1,1
%A A206447 _Jaroslav Krizek_, Feb 07 2012