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A068391
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Numbers n such that sigma(n) = 3*phi(n).
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7
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2, 15, 357, 3339, 5049, 10659, 12441, 24969, 99693, 124355, 132957, 145145, 353133, 423657, 596037, 655707, 734517, 745503, 894387, 1406427, 1641783, 1823877, 1936557, 3295047, 4108401, 4194183, 4776201, 5574699, 5842137, 5971251, 6132789, 6953765, 7649915
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OFFSET
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1,1
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COMMENTS
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If m>1 and 2*3^m-1 is prime then n=7*3^(m-1)*(2*3^m-1) is in the sequence.
Because sigma(n)=8*(3^m-1)/2*(2*3^m)=8*3^m*(3^m-1)=3*6*(2*3^(m-2))*(2*3^m-2) =3*phi(7)*phi(3^(m-1))*phi(2*3^m-1))=3*phi(7*3^(m-1)*(2*3^m-1))=3*phi(n). (End)
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LINKS
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MATHEMATICA
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Select[Range[765*10^4], DivisorSigma[1, #]==3EulerPhi[#]&] (* Harvey P. Dale, Aug 25 2019 *)
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PROG
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(PARI) for(n=1, 500000, if(sigma(n)==3*eulerphi(n), print1(n, ", ")))
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CROSSREFS
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Subsequence of A087943 (sigma(k) is a multiple of 3).
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KEYWORD
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easy,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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