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A015704
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a(n) is the smallest number m such that phi(m) + sigma(m) = n*m.
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4
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OFFSET
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2,2
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COMMENTS
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The offset is 2, because for all numbers m, phi(m)+sigma(m) >= 2*m, so there is no number a(1) such that phi(a(1))+sigma(a(1))=1*a(1). - Farideh Firoozbakht, Jan 22 2008
10^13 < a(5) <= 336280120525440. Charles R Greathouse IV showed that 6 divides a(5). 336280120525440 and 60493590969525342720 are the only m values I found such that phi(m) + sigma(m) = 5*m. - Donovan Johnson, Sep 11 2012
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LINKS
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PROG
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(PARI) a(n) = my(m = 1); while(sigma(m)+eulerphi(m) != n*m, m++); m; \\ Michel Marcus, Oct 04 2017
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CROSSREFS
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KEYWORD
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nonn,hard,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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