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A055234
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Smallest x such that sigma(x) = n*phi(x), or -1 if no such x exists.
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12
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1, 3, 2, 14, 56, 6, 12, 42, 30, 168, 2580, 210, 630, 420, 840, 20790, 416640, 9240, 291060, 83160, 120120, 5165160, 1719277560, 43825320, 26860680, 277560360, 1304863560, 569729160, 587133466920, 16522145640, 33044291280, 563462139240, 1140028049160, 9015394227840, 1255683068640, 65361608151840
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OFFSET
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1,2
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COMMENTS
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a(34) <= 9015394227840, a(35) <= 1255683068640. - Giovanni Resta, May 08 2017
Terms after a(36) are > 10^14. a(37) <= 4771397395084320, a(38) <= 2418379501618080, a(39) <= 413956851628320, a(40) <= 1241870554884960, and a(42) <= 50916692750283360. - Jud McCranie, Sep 13 2017
a(38) = 299761858075680, a(39) = 413956851628320. a(37), a(40), and higher terms are > 4.2*10^14. - Jud McCranie, Nov 27 2017
a(37), a(40), and higher terms are > 6.0 x 10^14. - Jud McCranie, Dec 27 2017
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LINKS
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FORMULA
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EXAMPLE
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sigma(14) = 24 = 4*phi(14), so a(4) = 14.
n = 21: a(21) = 120120 = 2*2*2*3*5*7*11*13, sigma(120120) = 483840 = n*phi(120120), phi(120120) = 23040.
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MATHEMATICA
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a[n_]:=(For[m=1, DivisorSigma[1, m]!=n EulerPhi[m], m++ ]; m); Do[Print[a[n]], {n, 31}] (* Farideh Firoozbakht, Oct 31 2008 *)
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PROG
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(PARI) a(n) = {k = 1; while(sigma(k) != n*eulerphi(k), k++); k; } \\ Michel Marcus, Sep 01 2014
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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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STATUS
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approved
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