[EM] Proportional preferential voting

[Markus Schulze]

Dear Craig,

you wrote (15 Sep 1999):
> 'permuting all preferences before a preference for a
> particular candidate, never makes a difference to
> that candidate's win/lose status'.

You wrote (18 Sep 1999):
> Principle 1
> Alterations of preferences after a preference
> for a winning candidate never cause that
> candidate to lose.

You wrote (18 Sep 1999):
> New definition:
>
> Principle 1 (P1), Sat 18 Sept 1999
>
> For all c (c is a candidate), all V, all V' (where
> V and V' are election systems), then if
> V' in AltAtAfter(V,c) and c loses V, then c
> also loses V'.
>
> AltAtAfter(V,c) is defined to be the set of all
> election papers collections that can be derived
> from V by altering preferences at and/or after the
> preference for preference c.
>
> For example, 
>
> V:
>  10 ABC
>  11 B
>  S
>
> One system in AltAtAfter(V,'B') is this:
> 10 A
>  2 AB
>  3 ACB
>  4 AD
>  1 B
>  1 C
>  S
>
> No deletion of the ABC papers was possible, so
> since A has 19, 9 papers did come from the alteration
> of the 11 B preferential voting papers.

You wrote (18 Sep 1999):
> I have a question for Mr Catchpole and Mr Markus Schulze,
> or anybody who wishes to answer:
> Does Condorcet (1 winner) satisfy the 'principle 1'
> (given above) ??.
> Please respond with a proof or a counter example.

As you ask me that directly, I have to answer that it seems
to me that only strategy-proof preferential election methods
meet both of your alteration rules simultaneously.

Markus Schulze