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A054571 a(n) = phi(n - phi(n)), a(1) = 0.

%I A054571 #18 Dec 27 2023 12:19:12
%S A054571 0,1,1,1,1,2,1,2,2,2,1,4,1,4,6,4,1,4,1,4,6,4,1,8,4,6,6,8,1,10,1,8,12,
%T A054571 6,10,8,1,8,8,8,1,8,1,8,12,8,1,16,6,8,18,12,1,12,8,16,12,8,1,20,1,16,
%U A054571 18,16,16,22,1,12,20,22,1,16,1,18,24,16,16,18,1,16,18
%N A054571 a(n) = phi(n - phi(n)), a(1) = 0.
%H A054571 Antti Karttunen, <a href="/A054571/b054571.txt">Table of n, a(n) for n = 1..10000</a>
%F A054571 a(1) = 0, and for n > 1, a(n) = totient(cototient(n)) = A000010(A051953(n)). - _Antti Karttunen_, Aug 07 2017
%p A054571 A051953 := proc(n)
%p A054571         n-numtheory[phi](n) ;
%p A054571 end proc:
%p A054571 A054571 := proc(n)
%p A054571         numtheory[phi](A051953(n)) ;
%p A054571 end proc: # _R. J. Mathar_, Oct 13 2011
%t A054571 Array[EulerPhi[# - EulerPhi@ #] &, 81] (* _Michael De Vlieger_, Aug 07 2017 *)
%o A054571 (PARI) A054571(n) = if(1==n,0,eulerphi(n - eulerphi(n))); \\ _Antti Karttunen_, Aug 07 2017
%Y A054571 Cf. A000010, A051953, A070556, A290087.
%K A054571 nonn
%O A054571 1,6
%A A054571 J. Sandor (mstaicu(AT)dualnet.ro), Mar 09 2002
%E A054571 Description clarified with a(1) = 0 explicitly set by convention. - _Antti Karttunen_, Aug 07 2017