[EM] Getting support levels and a social ordering from DAC/DSC
Kristofer Munsterhjelm
km_elmet at t-online.de
Tue Jul 10 04:49:21 PDT 2018
> Suppose X is the winner of DAC/DSC by the ordinary rules. Since X is the
> winner, and not Y, that means that the only non-X coalitions we
> encountered were ones that would have emptied the eligibles set. Suppose
> the coalition that reduced the eligibles set to X had support s_X. Then
> there exists some subset of coalitions, all with support greater than or
> equal to s_X, that reduced the eligibles set to X. However, there does
> not, for any other candidate Y, exist a subset of coalitions of support
> greater than s_X that reduces the eligibles set to Y; if there were,
> then Y would have been the winner, not X. So any other candidate Y has
> support level at most s_X. Hence X is also a winner using the
> per-candidate procedure above.
>
> If there's no way of tiebreaking equal support coalitions that would
> cause the DAC/DSC winner by ordinary rules to be Y, then there exists no
> subset of coalitions of support >= s_X that reduces the eligibles set to
> Y, and X is the unique winner using the per-candidate procedure above.
Now that I think about it, that could well be wrong. Consider something like
100: ABC
80: BC
20: A
19: B
10: C
The standard winner is B (intersect ABC, intersect BC, skip A, intersect
B), but the support-level check will falsely say A has support 20 here
against B's 19.
That's unfortunate. Is there any way to salvage the reasoning? It
doesn't seem obvious what A's score should be in the social ordering.
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