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%I #11 Jun 18 2017 02:00:50
%S 4,15,21,26,27,28,33,38,48,50,57,62,69,72,74,80,85,93,99,106,129,133,
%T 134,145,156,166,176,177,178,200,205,207,213,217,218,226,237,249,253,
%U 254,262,265,276,278,308,309,314,348,362,364,368,380,393,398,410,417
%N Numbers n for which tau(n) = tau(n'), where tau is the number of divisors of n and n' the arithmetic derivative of n.
%H Paolo P. Lava, <a href="/A209870/b209870.txt">Table of n, a(n) for n = 1..1000</a>
%e Divisors of 26 are four: 1, 2, 13 and 26. Arithmetic derivative of 26 is 15 and its divisors are four: 1, 3, 5 and 15.
%p with(numtheory);
%p A209870:=proc(n)
%p local a,i,p,pfs;
%p for i from 1 to n do
%p pfs:=ifactors(i)[2]; a:=i*add(op(2,p)/op(1,p),p=pfs);
%p if tau(a)=tau(i) then print(i); fi;
%p od; end:
%p A209870(1000);
%Y Cf. A000005, A003415, A190402, A190403
%K nonn,easy
%O 1,1
%A _Paolo P. Lava_, Mar 15 2012
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