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A373435
Iterate the function x <- phi(sigma(x)). The sequence has the smallest member of cycles of length 2.
10
4, 48, 72, 432, 1728, 10368, 184320, 1658880, 6220800, 10222080, 12856320000
OFFSET
1,1
COMMENTS
A cycle of length 2 also starts at 3852635996160. 3852635996160, 4869303828480, and 23971865863680 are also terms in the sequence. The sequence is complete through 10^13. - Jud McCranie, Sep 14 2024
EXAMPLE
phi(sigma(4)) = 6 and phi(sigma(6)) = 4, so 4 (the smallest term) is in the sequence.
MATHEMATICA
Select[Range[10^6], # == EulerPhi[DivisorSigma[1, EulerPhi[DivisorSigma[1, #]]]] && # < EulerPhi[DivisorSigma[1, #]]&] (* Stefano Spezia, Jun 07 2024 *)
PROG
(PARI) isok(x) = my(y = eulerphi(sigma(x))); if (y > x, x == eulerphi(sigma(y))); \\ Michel Marcus, Jun 06 2024
CROSSREFS
Subsequence of A067883. A067883 is a supersequence of this sequence.
Sequence in context: A242225 A157818 A362402 * A048608 A366492 A275033
KEYWORD
nonn,more
AUTHOR
Jud McCranie, Jun 06 2024
STATUS
approved