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A166374
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Numbers whose arithmetic derivative is equal to Euler totient function: n' = phi(n).
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12
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2, 9, 15, 625, 1225, 3993, 117649, 218491, 857375, 3788435, 4259571, 69302975, 136410197, 200533921, 313742585, 603439225, 1516358753, 2563893625, 3326174929, 5655792025, 10214476341, 25937424601, 29677977573, 59797108943, 283867750439, 715167055525
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OFFSET
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1,1
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COMMENTS
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The sequence is infinite. If n=prod(pi^ei) with each pi prime, then phi(n) = n*prod((pi-1)/pi) and n' = n*sum(ei/pi). Thus every number of the form p^(p-1), where p is prime, is in this sequence. - Nathaniel Johnston, Nov 27 2010
If p > q are primes and q does not divide p-1, there is a solution in positive integers of (p-1)*(q-1) = a*p + b*q, and then p^b*q^a is in the sequence. - Robert Israel, Aug 21 2014
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LINKS
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MAPLE
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A003415:= n -> n*add(f[2]/f[1], f=ifactors(n)[2]):
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MATHEMATICA
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(*Run the Mathematica program given in A003415 first, to define the function a as the arithmetic derivative.*) Select[Range[0, 10000], EulerPhi[ # ] == a[ # ] &]
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PROG
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(Python)
from sympy import factorint, totient
A166374 = [n for n in range(1, 10**6) if sum([int(n*e/p) for p, e in factorint(n).items()]) == totient(n)] # Chai Wah Wu, Aug 22 2014, edited by Antti Karttunen, Mar 13 2021
(Sage)
A166374_list = lambda n: filter(lambda k: euler_phi(k) == A003415(k), range(n))
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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Term a(1)=0 removed and the indices in the above comments decremented by one. - Antti Karttunen, Mar 13 2021
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STATUS
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approved
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