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A330701
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Numbers k such that psi(phi(k)) = 2 * phi(psi(k)), where psi(k) is the Dedekind psi function (A001615) and phi(k) is the Euler totient function (A000010).
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1
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26, 39, 45, 51, 52, 58, 74, 82, 98, 104, 111, 116, 135, 142, 146, 147, 148, 164, 178, 195, 196, 208, 219, 232, 284, 286, 292, 296, 328, 356, 357, 386, 392, 405, 406, 416, 435, 464, 495, 555, 561, 568, 572, 574, 579, 584, 585, 592, 598, 615, 622, 638, 646, 650
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OFFSET
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1,1
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COMMENTS
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Sandor proved that this sequence is infinite.
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LINKS
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EXAMPLE
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26 is in the sequence since psi(phi(26)) = psi(12) = 24, and 2 * phi(psi(26)) = 2 * phi(42) = 2 * 12 = 24.
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MATHEMATICA
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psi[1] = 1; psi[n_] := n * Times @@ (1 + 1/Transpose[FactorInteger[n]][[1]]); Select[Range[1000], psi[EulerPhi[#]] == 2 * EulerPhi[psi[#]] &]
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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