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%I #18 Dec 15 2017 17:35:36
%S 2,3,9,33,363,1089,1653,3237,4297761,32348343651
%N Numbers n such that sf(n) = phi(n), where sf(n) = A013928(n).
%C No more terms < 30000. All terms, except the first, are multiples of 3. - _Emeric Deutsch_, Jun 25 2005
%C a(11) > 5*10^10. - _Donovan Johnson_, Jun 10 2011
%C No other terms < 10^12. - _Jud McCranie_, Aug 27 2013
%e sf(363) = 220, phi(363) = 220.
%p with(numtheory): a:=proc(n) if add(mobius(k)^2,k=1..n-1)=phi(n) then n else fi end: seq(a(n),n=1..5000); # _Emeric Deutsch_, Jun 25 2005
%o (PARI) sf(n)=s=0; for(x=1,n, if(issquarefree(x),s++)); s; for(n=1,10^5, if(sf(n)==eulerphi(n),print(n)))
%K more,nonn
%O 1,1
%A _Jason Earls_, Sep 07 2001
%E Corrected and extended by _Emeric Deutsch_, Jun 25 2005
%E a(9) from _Donovan Johnson_, Jul 15 2009
%E a(10) from _Donovan Johnson_, Jun 10 2011
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