cp's OEIS Frontend

A173746 Numbers k such that tau(tau(k)) = rad(k).

%I A173746 #15 May 10 2019 22:38:08
%S A173746 1,2,4,16,27,64,72,96,108,288,432,486,648,768,972,1024,1536,1728,3456,
%T A173746 4096,5832,6561,13122,17496,20736,24576,27648,39366,41472,65536,98304,
%U A173746 104976,110592,147456,186624,256000,262144,314928,400000,419904,472392
%N A173746 Numbers k such that tau(tau(k)) = rad(k).
%C A173746 Tau = A000005 is the number of divisors of its argument. rad(n) = A007947(n) is the product of the primes dividing n.
%C A173746 Note that rad() is idempotent: rad(rad(n)) = rad(n). - _R. J. Mathar_, Nov 07 2011
%H A173746 Donovan Johnson, <a href="/A173746/b173746.txt">Table of n, a(n) for n = 1..496</a> (terms < 10^18)
%F A173746 {n : A010553(n) = A007947(n)}.
%e A173746 288 is in the sequence because tau(288)= 18, tau(18)=6, rad(288)=6.
%p A173746 A010553 := proc(n)
%p A173746         numtheory[tau](numtheory[tau](n)) ;
%p A173746 end proc:
%p A173746 for n from 1 to 480000 do
%p A173746         if A010553(n) = A007947(n) then
%p A173746                 printf("%d,",n) ;
%p A173746         end if;
%p A173746 end do: # _R. J. Mathar_, Nov 07 2011
%K A173746 nonn
%O A173746 1,2
%A A173746 _Michel Lagneau_, Feb 23 2010
%E A173746 Example corrected and edited by _Michel Lagneau_, Apr 25 2010