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A068581
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Let phi_m(x) = phi(phi(...(phi(x))...)) m times; sequence gives values of k such that phi_4(k) = tau(k).
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4
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1, 17, 23, 29, 31, 37, 43, 51, 55, 65, 69, 77, 82, 87, 91, 93, 94, 95, 106, 111, 118, 122, 128, 129, 133, 134, 136, 142, 146, 158, 165, 170, 184, 195, 218, 230, 231, 232, 238, 243, 246, 248, 250, 254, 273, 282, 285, 286, 290, 296, 297, 310, 318, 322, 344, 351
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OFFSET
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1,2
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COMMENTS
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Last term is a(132) = 7560.
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LINKS
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MATHEMATICA
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Select[Range[351], Nest[EulerPhi, #, 4] === DivisorSigma[0, #] &] (* Amiram Eldar, Jun 12 2022 *)
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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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