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A068580
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Let phi_m(x) = phi(phi(...(phi(x))...)) m times; sequence gives values of k such that phi_2(k) = tau(k).
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3
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1, 5, 7, 15, 21, 22, 26, 40, 56, 66, 70, 78, 108, 120, 126, 168, 210
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OFFSET
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1,2
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COMMENTS
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LINKS
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MATHEMATICA
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Select[Range[210], Nest[EulerPhi, #, 2] === DivisorSigma[0, #] &] (* Amiram Eldar, Jun 12 2022 *)
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PROG
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(PARI) is(k) = numdiv(k) == eulerphi(eulerphi(k)); \\ Jinyuan Wang, Apr 05 2020
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CROSSREFS
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KEYWORD
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nonn,easy,fini,full
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AUTHOR
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STATUS
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approved
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