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A069552
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Numbers k such that phi(k) = sigma(core(k)) where phi(k) is the Euler totient function, sigma(k) the sum of divisors of k and core(k) the squarefree part of k (the smallest integer such that k*core(k) is a square).
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1
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1, 12, 56, 140, 270, 630, 1672, 4180, 6426, 18810, 80104, 93496, 99484, 102856, 116116, 140296, 191862, 200260, 220616, 223938, 224536, 233740, 257140, 350740, 447678, 449442, 522522, 551540, 561340, 702240, 901170, 1051830, 1157130, 1578330, 2481930, 2526030
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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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EXAMPLE
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140 is in the sequence as phi(140) = 48 = sigma(35) = sigma(core(140)). - David A. Corneth, Sep 08 2020
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MATHEMATICA
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core[n_] := Times @@ Power @@@ ({#[[1]], Mod[ #[[2]], 2]} & /@ FactorInteger[n]); Select[Range[10^5], EulerPhi[#] == DivisorSigma[1, core[#]] &] (* Amiram Eldar, Jul 11 2019 after Zak Seidov at A007913 *)
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PROG
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(PARI) for(n=1, 10^6, if(eulerphi(n)==sigma(core(n)), print1(n, ", ")))
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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