Is a 2x2 Complex Matrix with N^2=0 Always Similar to a Specific Form?

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Discussion Overview

The discussion centers on the properties of a 2x2 complex matrix N such that N^2=0. Participants explore whether such a matrix is either the zero matrix or similar to a specific form, engaging in theoretical reasoning and mathematical exploration.

Discussion Character

  • Exploratory
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that if N is not zero, there exists a vector v such that Nv is not zero, leading to a basis of C^2 formed by v and Nv.
  • Another participant confirms the linear independence of v and Nv, proposing to express N in terms of this basis.
  • A participant describes their attempt to construct the matrix representation of N in the basis (v, Nv) and questions whether the first coordinate x1 must be zero.
  • Further clarification is provided on how to write N as a matrix with respect to the chosen basis, emphasizing the implications of N^2=0.
  • There is a mention of the concept of similarity of matrices, noting that matrices remain self-similar under change of basis.

Areas of Agreement / Disagreement

Participants appear to agree on the linear independence of the vectors involved and the approach to expressing N in a specific basis. However, there is uncertainty regarding the implications of the coordinates in the matrix representation and the broader question of similarity.

Contextual Notes

Participants have not resolved whether x1 must be zero, and there are unresolved steps regarding the demonstration of similarity of matrices.

Alupsaiu
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Suppose N is a 2x2 complex matrix such that N^2=0. Prove that either N=0 or N is similar over C to the matrix

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10Sorry, I don't know how else to write the matrix in the post. Any help would be greatly appreciated, thank you.
 
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If N is not zero, then it exists a vector v such that Nv is not zero. Using the fact that N^2=0 it is easy to proove that v and Nv are linearly independent (try it), so they are a basis of C^2. Try writing N with respect to this basis.
 
Ok, so it was easy to show linear independence, then I found the matrix which had in the first column the coordinates of Nv, and in the second column 0 since N^2=0.
So,

x1 0
x2 0, where x1 and x2 are the coordinates of Nv in basis (v, Nv). Would x1 actually turn out to be 0?


But, where to go from here? In general I'm a bit confused on how to show a matrix is similar, or must be similar to another matrix. Again, thank you for the help.
 
If {v, Nv} is your basis, then writing N as a matrix with respect to this basis would be of the form [N(v) N(Nv)] where N(v) and N(Nv) are column vectors. If you use the fact that N^2= 0, the rest will follow.

As for showing similarity, recall that matrices are self similar even after change of basis.
 

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