I'm stuck on a proof that's probably trivial, any insights?

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Discussion Overview

The discussion revolves around proving a relationship between a polynomial defined by variables raised to their respective powers and the Vandermonde determinant, particularly in the context of determinants and the symmetric group. The scope includes mathematical reasoning and exploration of definitions related to determinants.

Discussion Character

  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant presents a polynomial \( P(a,b,c,d,e,f...n) = (a^0)(b^1)(c^2)...(n^n) \) and seeks to show that the sum of this polynomial acted upon by the symmetric group, multiplied by the sign function, equals the Vandermonde determinant.
  • Another participant questions the definition of the determinant being used, specifically mentioning the definition that incorporates the sign function.
  • A participant asserts they are using the standard definition of the determinant from co-factor expansion but acknowledges a simpler formula for the Vandermonde determinant.
  • There is a mention of a general identity that the identity being proved is a special case of, suggesting a broader context for the discussion.
  • One participant expresses confusion about connecting the sum of alternating polynomials with the definition of the Vandermonde determinant.
  • Several participants reference external resources, including Wikipedia and class notes, to clarify definitions and properties related to determinants.

Areas of Agreement / Disagreement

Participants express differing levels of understanding regarding the connection between the polynomial sum and the Vandermonde determinant. While some participants seem to agree on the general relationship, the discussion remains unresolved regarding the specific proof and definitions involved.

Contextual Notes

There are limitations in the discussion regarding the clarity of definitions used for determinants and the assumptions underlying the polynomial's behavior under the symmetric group. The connection between the polynomial sum and the Vandermonde determinant is not fully established, leaving open questions about the proof process.

PsychonautQQ
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Given polynomial P(a,b,c,d,e,f...n) = (a^0)(b^1)(c^2)...(n^n)

show that the sum of P(a,b,c,d,e,f...x) when acted upon by the symmetric group of order n and each time multipled by the sgn function (1 if even and -1 if odd); that this sum is equal to the vandermonde determinant of these variables. Any insights? I'm quite lost.
 
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What is your definition of the determinant? Do you know of the definition which uses the sign function?
 
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Well, I believe that I'm using the standard definition for the determinant of the matrix (the resulting polynomial from co-factor expansion). However, the resulting determinant from a vandermonde matrix can be given by a simpler formula; it's given on this page under 'properties' https://en.wikipedia.org/wiki/Vandermonde_matrix.
 
So yes, I'm using the definition that involves the sign function. I'm still having trouble getting from the sum of alternating polynomials acted on by the symmetric group and the definition of the determinant of the vandermonde matrix.
 
Wow, so this is like exactly what I'm looking for?
 
Yep!
 
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u da boss :D
 
So just to clarify, what you're saying is that the map that I'm trying to prove is equivalent to the vandermonde determinant is really equivalent to determinants in general, and thus equivalent to the vandermonde determinant.
 
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Yes. The identity you are trying to prove is a special case of a more general identity.
 
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If you want to go through the details of this standard permutation approach to determinants, including the proof that it agrees with the expansion by rows you mention, it is explained on pages 62-67 of these class notes:

http://alpha.math.uga.edu/%7Eroy/4050sum08.pdf
 
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