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%I #7 Sep 20 2016 13:18:40
%S 1,3,6,20259,43912,563103,18895035,119847148,305478634710
%N Numbers n>0 such that n’ = (n-1)’ + (n-2)’, where n’ is the arithmetic derivative of n.
%C a(10) > 5*10^12. - _Giovanni Resta_, Sep 16 2016
%e 6’ = 5, 5’ = 1, 4’ = 4 and 5 = 1 + 4.
%p with(numtheory): P:= proc(q) local a,b,c,n,p;
%p a:=n*add(op(2,p)/op(1,p),p=ifactors(n)[2]);
%p b:=(n-1)*add(op(2,p)/op(1,p),p=ifactors(n-1)[2]);
%p c:=(n-2)*add(op(2,p)/op(1,p),p=ifactors(n-2)[2]);
%p if a=b+c then print(n); fi; od; end: P(10^9);
%Y Cf. A003415.
%K nonn,more
%O 1,2
%A _Paolo P. Lava_, Sep 15 2016
%E a(9) from _Giovanni Resta_, Sep 16 2016
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