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A306478
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The phi-radical numbers: composite numbers m such that rad(phi(m)) = rad(m-1).
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1
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1729, 2431, 6601, 9605, 10585, 12801, 15211, 30889, 46657, 69751, 88561, 92929, 105001, 159895, 272323, 368641, 460801, 534061, 610051, 622909, 950797, 992251, 1047619, 1372801, 1374895, 1745701, 1902691, 2210671, 2628073, 2704801, 3225601, 5629339, 5690251, 6840001, 9738751
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OFFSET
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1,1
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COMMENTS
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Problem: are there infinitely many such numbers?
These numbers are odd squarefree. They contain many Carmichael numbers.
The smallest such semiprime is 1525781251 = 19531*78121, see A306479.
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LINKS
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MATHEMATICA
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rad[n_] := Times @@ (First@# & /@ FactorInteger@n); Select[Range[100000], CompositeQ[#] && rad[EulerPhi[#]] == rad[# - 1] &]
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PROG
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(PARI) rad(n) = factorback(factorint(n)[, 1]); \\ A007947
isok(m) = !isprime(m) && (rad(eulerphi(m)) == rad(m-1)); \\ Michel Marcus, Feb 18 2019
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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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