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 A194085 Strong Achilles numbers: Achilles numbers m such that phi(m) is also an Achilles number, where phi(m) denotes Euler's totient function of m. 1

%I

%S 500,864,1944,2000,2592,3456,5000,10125,10368,12348,12500,16875,19652,

%T 19773,30375,31104,32000,33275,37044,40500,49392,50000,52488,55296,

%U 61731,64827,67500,69984,78608,80000,81000,83349,84375,93312,108000

%N Strong Achilles numbers: Achilles numbers m such that phi(m) is also an Achilles number, where phi(m) denotes Euler's totient function of m.

%H Ray Chandler, <a href="/A194085/b194085.txt">Table of n, a(n) for n = 1..656</a>

%H Project Euler, <a href="https://projecteuler.net/index.php?section=problems&amp;id=302">Problem 302: Strong Achilles Numbers</a>

%t achillesQ[n_] := With[{ee = FactorInteger[n][[All, 2]]}, Min[ee] > 1 && GCD @@ ee == 1];

%t Select[Range[10^6], achillesQ[#] && achillesQ[EulerPhi[#]]&] (* _Jean-François Alcover_, Sep 26 2020 *)

%o (PARI) isA(n)=!ispower(n) && ispowerful(n)

%o is(n)=isA(n) && isA(eulerphi(n)) \\ _Charles R Greathouse IV_, Dec 19 2013

%Y Cf. A052486 (Achilles numbers)

%K nonn

%O 1,1

%A _Kausthub Gudipati_, Aug 14 2011

%E Extended by _Ray Chandler_, Aug 16 2011

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Last modified April 16 10:22 EDT 2021. Contains 343036 sequences. (Running on oeis4.)