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A173327
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Numbers k such that tau(phi(k))= sopf(k).
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2
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4, 45, 48, 75, 160, 180, 252, 294, 300, 315, 336, 351, 378, 396, 475, 507, 560, 605, 616, 650, 833, 936, 1216, 1375, 1452, 1690, 1805, 1920, 2023, 2112, 2200, 2349, 2496, 2736, 3211, 3520, 3648, 4095, 4160, 4256, 4332, 4389, 4464, 4477, 4508, 4620, 4693
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OFFSET
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1,1
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COMMENTS
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tau(k) is the number of divisors of k (A000005); phi(k) is the Euler totient function (A000010); and sopf(k) is the sum of the distinct primes dividing k without repetition (A008472).
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REFERENCES
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M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, 1964 (and various reprintings), p. 840.
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LINKS
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FORMULA
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EXAMPLE
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4 is in the sequence because phi(4) = 2, tau(2)=2 and sopf(4)=2 ;
45 is in the sequence because phi(45) = 24, tau(24)=8 and sopf(45)=8.
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MAPLE
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for n from 1 to 150000 do : t1:= ifactors(n)[2] : t2 :=sum(t1[i][1], i=1..nops(t1)):if tau(phi(n)) = t2 then print (n): else fi : od :
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MATHEMATICA
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tpsQ[n_]:=DivisorSigma[0, EulerPhi[n]]==Total[Transpose[FactorInteger[n]][[1]]]; Select[Range[5000], tpsQ] (* Harvey P. Dale, Apr 10 2013 *)
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PROG
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(PARI) sopf(n)=my(f=factor(n)[1, ]); sum(i=1, #f, f[i])
(Magma) [ m:m in [2..5100]|#Divisors(EulerPhi(m)) eq &+PrimeDivisors(m)]; // Marius A. Burtea, Jul 10 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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EXTENSIONS
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Added punctuation to the examples. Corrected and edited by Michel Lagneau, Apr 25 2010
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STATUS
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approved
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