This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A091911 #5 Mar 30 2012 19:01:05
%S A091911 1,6,28,0,140,270,8128,672,1638,6200,2970,0,105664,18620,8190,0,27846,
%T A091911 0,117800,0,55860,0,0,30240,173600,242060,167400,0,237510,0,
%U A091911 2305843008139952128,0,0,0,2229500,0,4358600,0,726180,0,2290260,1089270
%N A091911 a(n) = smallest number m such that m*tau(m)/sigma(m) = n, or 0 if no such m exists.
%C A091911 Assumes that A090240 is correct.
%t A091911 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
%Y A091911 The zeros occur at the complement of A090240.
%K A091911 nonn
%O A091911 1,2
%A A091911 _Robert G. Wilson v_ and _R. K. Guy_, Feb 11 2004