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A062699
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Numbers n such that sigma(n) = 2*phi(n).
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20
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3, 35, 1045, 24871, 29029, 50065, 58435, 64285, 87685, 137885, 140335, 1390753, 1529983, 1739507, 2011009, 2086903, 3189625, 3281663, 3501605, 3722875, 3830827, 3852155, 6605945, 7711405, 8409305, 9815195, 11413205, 11569805, 13321295, 13932919, 16540205
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OFFSET
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1,1
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COMMENTS
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3 is the only prime term of this sequence. There is no term of the form p^k where p is a prime and k>1. All terms are odd because if n is even then 2*phi(n)=phi(2n)<=n<sigma(n). - Farideh Firoozbakht, Apr 01 2005, Feb 24 2007
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LINKS
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MATHEMATICA
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Select[Range[10^6], DivisorSigma[1, #] == 2 * EulerPhi[#] &] (* Amiram Eldar, Dec 04 2019 *)
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PROG
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(PARI) for(n=1, 500000, if(sigma(n)==eulerphi(n)*2, print(n)))
(PARI) n=0; for (m=1, 10^9, if(sigma(m)==2*eulerphi(m), write("b062699.txt", n++, " ", m); if (n==50, break)) ) \\ Harry J. Smith, Aug 09 2009
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CROSSREFS
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Subsequence of A028983 (sigma(k) is even).
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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