How to construct a vector orthogonal to all but one?

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    Orthogonal Vector
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Discussion Overview

The discussion revolves around the problem of constructing a vector that is orthogonal to a set of linearly independent vectors, specifically all but one vector from the set. The context includes theoretical approaches and methods such as the Gram-Schmidt process and alternative brute-force methods.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant inquires about constructing a vector orthogonal to all vectors except one, suggesting the use of the Gram-Schmidt process or brute-force methods.
  • Another participant agrees that the discussion is in the context of Rn and considers the Gram-Schmidt process to be a form of brute force.
  • A participant questions the necessity of the Gram-Schmidt process, proposing a brute-force method of solving n-1 equations instead.
  • There is curiosity expressed about whether a neat expression for the orthogonal vector exists.
  • One participant reflects on the lack of prior knowledge about the other vectors and suggests that orthogonalizing is necessary to eliminate components of the vector of interest.
  • Another participant argues that Gram-Schmidt is not essential and proposes a modified version of the process focused solely on the vector of interest, while also acknowledging the validity of the brute-force method.
  • A later reply suggests a potential expression for the orthogonal vector, although it is not fully elaborated.

Areas of Agreement / Disagreement

Participants express differing opinions on the necessity and efficiency of the Gram-Schmidt process versus brute-force methods. There is no consensus on the best approach to construct the desired orthogonal vector.

Contextual Notes

Participants mention the need to solve equations related to dot products, but the specifics of these equations and their implications remain unresolved. The discussion also highlights the dependence on the definitions and properties of the vectors involved.

td21
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Given n linearly independent vectors, v1, v2, v3, ...vn.
How to find construct a vector that is orthogonal to v2, v3, ..., vn (all v but not v1)?
Is Gram Schmitt process the way to do this? or just by brute force?
 
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I take it this is in Rn ? Then: yes !
Gram Schidt is brute force as far as I am concerned.
 
BvU said:
I take it this is in Rn ? Then: yes !
Gram Schidt is brute force as far as I am concerned.
Thanks for the reply. But why is Gram Schmidt process needed? My original brute force idea is to solve n-1 equations.
 
I also wonder if there is a neat expression for such vector?:oldsmile:
 
Ah, sorry, I didn't read carefully enough. You have no prior knowledge of the vv ... vn to exploit, so orthogonalizing seems to me the only way to get rid of the components of v1 that are in the subspace spanned by vv ... vn ... But by now I'm not all that certain any more...
 
Gram-Schmidt is not necessary. People usually think of Gram-Schmidt as making an entire set of n orthogonal vectors. Is that what you mean?

Since you only care about finding the orthogonal part of v1, I think you can modify the Gram-Schmidt process to work on only v1. Just keep subtracting the projection of the next vi on what remains of v1.

Your "brute-force" method of solving equations also works. Just like the modified Gram-Schmidt, it does nothing to make the v2, ..., vn orthodonal. I don't know which approach would be less "brute-force".

PS. Don't you mean n equations? You want the dot product of x with v1 to be nonzero (set it = 1) and all the other dot products to be =0.
 
td21 said:
I also wonder if there is a neat expression for such vector?:oldsmile:
I think that would be w = [v1; v2; ... ; vn]-1(1, 0, 0, ..., 0)
 

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