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A067651
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Numbers n such that sigma(n) = phi(prime(n)-1).
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2
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1, 7, 19, 47, 115, 177, 280, 297, 340, 406, 470, 930, 1240, 1608, 1836, 2232, 2380, 2484, 4326, 8360, 33864, 166152, 216216, 220248, 299250, 394500
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OFFSET
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1,2
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COMMENTS
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No further term below 4*10^6.
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LINKS
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EXAMPLE
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sigma(19) = 20, prime(19) = 67, phi(66) = 20, hence 20 is a term.
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MATHEMATICA
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Select[Range[1, 100000],
DivisorSigma[1, #] == EulerPhi[Prime[#] - 1] &] (* Julien Kluge, Jul 08 2016 *)
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PROG
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(PARI) m=400000; for(n=1, m, if(sigma(n)==eulerphi(prime(n)-1), print1(n, ", ")))
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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