What Is the Inner Product <V,s> in Complex Vector Projections?

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Discussion Overview

The discussion revolves around the concept of projecting complex vectors, specifically focusing on the inner product denoted as in the context of a matrix of complex numbers V and a vector s. Participants explore the implications of this projection, the definitions of inner products, and the relationships between the dimensions of the involved matrices and vectors.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant seeks clarification on whether represents the inner product of the entire matrix V with the vector s or just the inner product of a specific column V(j) with s, noting that V or V(j) would be the Hermitian of the vector.
  • Another participant expresses unfamiliarity with the formula and questions the meaning of , as well as the projection of s onto V(j), highlighting that V(j) may not belong to the same vector space as s.
  • A participant clarifies their intent, stating that they are referring to the projection of vector s onto a column of matrix V, and reiterates their understanding of projections in the context of real numbers.
  • Another participant challenges the notion of taking the inner product of V and s, arguing that since V is an n×m matrix and s is a different dimension, they cannot be in the same vector space, thus complicating the definition of the inner product.
  • This participant also emphasizes that the term "vectors" can refer to more than just ordered sets of numbers and discusses the nature of vector spaces, particularly those involving complex numbers.

Areas of Agreement / Disagreement

Participants express differing views on the definition and applicability of the inner product in this context, with no consensus reached regarding the interpretation of or the feasibility of projecting s onto V(j).

Contextual Notes

There are unresolved issues regarding the dimensionality of the matrices and vectors involved, as well as the definitions of inner products in relation to complex vector spaces.

polaris90
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I need some clarification on projections of complex vectors. If I have a nxm matrix of complex numbers V and a mx1 matrix s, and I want to find the projection of s onto any column of V. The formula to do this is

c = <V, s>/||V(j)||^2 where V(j) is the column of V to be used. My question is, what is <V,s>? is that the inner product of the whole matrix V with s, or is it the inner product of V(j) with s? Where V or V(j) would be the Hermition of the vector.
 
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Where did you find this formula? I'm only familiar with projections of vectors onto other vectors and onto subspaces. I have no idea what <V,s> means, or what the projection of s onto V(j) means, since V(j) isn't a member of the same vector space as s.
 
I see I wasn't clear on my question. What I meant is the projection of vector vector s onto a vector V. By V(j) I meant a column of matrix V. <V, s> is the inner product of V and s. I know about projections of one vector onto another vector when they are all real numbers. In this case, I have a matrix V with complex numbers. I want to project s onto a column of V. I hope my question is clearer now.
 
But if V is an n×m matrix and s is not, how can you be talking about the inner product of V and s? An inner product takes two members of a vector space (the same vector space) to a number, but V and s aren't in the same vector space. Also, if s is m×1, and V(j) is n×1, they're not in the same vector space either (unless of course n=m).

Another thing: You seem to be thinking of "vectors" as ordered sets of numbers. That's not always the case. The members of any vector space are called vectors. A vector space V is considered "complex" when the scalar multiplication operation is a function from V×ℂ into V. There are real vector spaces whose members are matrices with complex entries.
 
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