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A273800 Numbers n for which n = phi(x)*phi(y), where n = x + y and phi(x) is the Euler totient function of x.

%I A273800 #14 Jun 07 2016 00:39:41
%S A273800 8,12,16,24,32,36,48,96,128,160,192,288,768,1152,2048,2560,3072,27648,
%T A273800 110592,192704,196608,202496,232448,370688,379904,394264,443512,
%U A273800 466048,508672,524288,553120,571008,586016,607744,624704,650624,655360,675584,681856
%N A273800 Numbers n for which n = phi(x)*phi(y), where n = x + y and phi(x) is the Euler totient function of x.
%H A273800 Giovanni Resta, <a href="/A273800/b273800.txt">Table of n, a(n) for n = 1..1000</a>
%e A273800 8 = 3+5 = phi(3)*phi(5) = 2*4;
%e A273800 12 = 3+9 = phi(3)*phi(9) = 2*6;
%e A273800 24 = 3+21 = phi(3)*phi(21) = 2*12 or 24 = 10+14 = phi(10)*phi(14) = 4*6.
%p A273800 with(numtheory): P:=proc(q) local a,b,k,n; for n from 1 to q do
%p A273800 for k from 1 to trunc(n/2) do if phi(k)*phi(n-k)=n then print(n); break; fi;
%p A273800 od; od; end: P(10^9);
%o A273800 (PARI) is(n)=for(x=1,n\2, if(eulerphi(x)+eulerphi(n-x)==n, return(1))); 0 \\ _Charles R Greathouse IV_, Jun 07 2016
%Y A273800 Cf. A000010, A273801.
%K A273800 nonn
%O A273800 1,1
%A A273800 _Paolo P. Lava_, May 31 2016
%E A273800 a(19)-a(39) from _Giovanni Resta_, May 31 2016