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%I #59 Aug 09 2024 03:27:54
%S 4,48,72,432,1728,10368,184320,1658880,6220800,10222080,12856320000
%N Iterate the function x <- phi(sigma(x)). The sequence has the smallest member of cycles of length 2.
%C A cycle of length 2 also starts at 3852635996160.
%e phi(sigma(4)) = 6 and phi(sigma(6)) = 4, so 4 (the smallest term) is in the sequence.
%t Select[Range[10^6], # == EulerPhi[DivisorSigma[1,EulerPhi[DivisorSigma[1,#]]]] && # < EulerPhi[DivisorSigma[1,#]]&] (* _Stefano Spezia_, Jun 07 2024 *)
%o (PARI) isok(x) = my(y = eulerphi(sigma(x))); if (y > x, x == eulerphi(sigma(y))); \\ _Michel Marcus_, Jun 06 2024
%Y Cf. A000010, A000203, A062401, A001229, A095955. A095956, A373453, A373454.
%Y Subsequence of A067883. A067883 is a supersequence of this sequence.
%K nonn,more
%O 1,1
%A _Jud McCranie_, Jun 06 2024
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