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 A091911 a(n) = smallest number m such that m*tau(m)/sigma(m) = n, or 0 if no such m exists. 1
 1, 6, 28, 0, 140, 270, 8128, 672, 1638, 6200, 2970, 0, 105664, 18620, 8190, 0, 27846, 0, 117800, 0, 55860, 0, 0, 30240, 173600, 242060, 167400, 0, 237510, 0, 2305843008139952128, 0, 0, 0, 2229500, 0, 4358600, 0, 726180, 0, 2290260, 1089270 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Assumes that A090240 is correct. LINKS MATHEMATICA f[n_] := (n*DivisorSigma[0, n]/DivisorSigma[1, n]); a = Table[0, {50}]; Do[ b = f[n]; If[ IntegerQ[b] && b < 51 && a[[b]] == 0, a[[b]] = n], {n, 1, 10 ^7}]; a CROSSREFS The zeros occur at the complement of A090240. Sequence in context: A006174 A064810 A101454 * A215896 A211679 A261868 Adjacent sequences:  A091908 A091909 A091910 * A091912 A091913 A091914 KEYWORD nonn AUTHOR Robert G. Wilson v and R. K. Guy, Feb 11 2004 STATUS approved

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Last modified February 21 02:57 EST 2019. Contains 320364 sequences. (Running on oeis4.)