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A341939 Numbers m such that phi(m)/tau(m) is a square of an integer where phi is the Euler totient function (A000010) and tau is the number of divisors function (A000005). 2

%I

%S 1,3,8,10,18,19,24,30,34,45,52,57,73,74,85,102,125,135,140,152,153,

%T 156,163,182,185,190,202,219,222,252,255,333,342,360,375,394,416,420,

%U 436,451,455,456,459,476,489,505,514,546,555,570,584,606,625,629,640,646,679,680,730

%N Numbers m such that phi(m)/tau(m) is a square of an integer where phi is the Euler totient function (A000010) and tau is the number of divisors function (A000005).

%C The first 11 terms of this sequence are also the first 11 terms of A341938: m such that phi(m)*tau(m) is a square, then, a(12) = 57 while A341938(12) = 54. Indeed, if phi(m)/tau(m) is a perfect square then phi(m)*tau(m) is also a square, but the converse is false. These counterexamples are in A341940, the first one is 54 (last example).

%C Some subsequences (see examples):

%C -> The seven terms that satisfy also tau(m) = phi(m) form the subsequence A020488 with phi(m)/tau(m) = 1^2.

%C -> Primes p of the form 2*k^2 + 1 (A090698) form another subsequence because tau(p) = 2 and phi(p) = p-1 = 2*k^2, so phi(p)/tau(p) = k^2.

%C -> Cubes p^3 where p is a prime of the form k^2+1 (A002496) form another subset because if p = 2, phi(8)/tau(8)=1, and if p odd, phi(p^3)/tau(p^3) = (k*p/2)^2 with k even.

%e phi(30) = 8, tau(30) = 8 so phi(30)/tau(30) = 1^2, and 30 is a term.

%e phi(45) = 24, tau(45) = 6, so phi(45)/tau(45) = 4 = 2^2, and 85 is a term.

%e phi(125) = 100, tau(125) = 4, so phi(125)/tau(125) = 25 = 5^2, and 125 is a term.

%e phi(54) = 18, tau(54) = 8, and phi(54)/tau(54) = 18/8 = 9/4 = (3/2)^2 and 54 is not a term while phi(54)*tau(54) = 12^2.

%p with(numtheory): filter:= q -> phi(q)/tau(q) = floor(phi(q)/tau(q)) and issqr(phi(q)/tau(q)) : select(filter, [$1..750]);

%t Select[Range[1000], IntegerQ @ Sqrt[EulerPhi[#]/DivisorSigma[0, #]] &] (* _Amiram Eldar_, Feb 24 2021 *)

%o (PARI) isok(m) = my(x=eulerphi(m)/numdiv(m)); (denominator(x)==1) && issquare(x); \\ _Michel Marcus_, Feb 24 2021

%Y Intersection of A020491 and A341938.

%Y Similar for: A144695 (sigma(n)/tau(n) perfect square), A293391 (sigma(n)/phi(n) perfect square).

%Y Subsequences: A090698, A020488.

%Y Cf. A069237, A289585, A341940.

%Y Cf. A000005 (phi), A000010(tau).

%K nonn

%O 1,2

%A _Bernard Schott_, Feb 24 2021

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Last modified June 25 10:28 EDT 2021. Contains 345453 sequences. (Running on oeis4.)