Is a Zero Row Necessary in the Square Root of a Zero Matrix?

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Discussion Overview

The discussion revolves around the properties of square roots of the zero matrix, specifically whether a square root must necessarily contain at least one zero row or column. Participants explore examples, counterexamples, and the implications of matrix properties in this context.

Discussion Character

  • Exploratory
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant initially believed no square matrix could square to the zero matrix but later proposed a counterexample that was incorrect, leading to questions about the necessity of zero rows or columns in such matrices.
  • Another participant suggested a different matrix that does square to the zero matrix, indicating that there are indeed matrices that fulfill this condition.
  • A further contribution discussed the implications of non-invertible matrices and the possibility of products equaling zero without either factor being zero.
  • Another participant noted the general lack of uniqueness in square roots of matrices, while also highlighting the uniqueness of the principal square root for positive definite matrices.
  • A participant expressed gratitude for the clarifications and questioned whether the previous posts imply that square roots of the zero matrix must be singular.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether a square root of the zero matrix must contain a zero row or column, and multiple competing views and examples are presented throughout the discussion.

Contextual Notes

There are unresolved assumptions regarding the definitions of square roots in the context of matrices, and the discussion includes various mathematical properties that may not be universally applicable.

Bipolarity
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At first I thought that there is no square matrix whose square is the 0 matrix. But I found a counterexample to this. My counterexample is:
[tex]\left( \begin{array}{cc} 0 & 0 \\ 0 & 1 \end{array} \right)[/tex]

However it appears that my counterexample has a 0 row. I'm curious, must a square root of the 0 matrix necessarily have at least one 0 row (or 0 column)?

BiP
 
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Bipolarity said:
At first I thought that there is no square matrix whose square is the 0 matrix. But I found a counterexample to this. My counterexample is:
[tex]\left( \begin{array}{cc} 0 & 0 \\ 0 & 1 \end{array} \right)[/tex]

However it appears that my counterexample has a 0 row. I'm curious, must a square root of the 0 matrix necessarily have at least one 0 row (or 0 column)?

BiP

The square of that matrix is the same matrix, not the zero matrix. Did you accidentally multiply when you should've added?
 
I suspect you intended the following matrix?
$$\begin{bmatrix}0 & 1 \\ 0 & 0 \end{bmatrix}$$
Square it and you get the zero matrix.

The same holds for
$$\begin{bmatrix}1 & 1 \\ -1 & -1 \end{bmatrix}$$
 
IF A2= 0 and A is invertible, then we could multiply both sides by A-1 and get A= 0. However, the ring of matrices as "non-invertible" matrices. It is quite possible to have AB= 0 with neither A nor B 0 and, in particular, non-zero A such that A2= 0.
 
The "square root of a matrix" isn't a very useful idea for general matrices, because it is hardly ever unique. See http://en.wikipedia.org/wiki/Square_root_of_a_matrix for the sort of (probably unexpected) things that can happen.

However the positive definite square root of a positive definite matrix (called its "principal square root") is unique, and sometimes useful.

If A is a symmetric matrix, finding B such that A = BB^T, is even more useful. B has most of the useful properties of the "square root or A", even when it is not a symmetric matrix.
 
Thank you all for your replies! Sorry for my mistake but I get it now!

HallsofIvy, does your post essentially prove that square roots of the 0 matrix must be singular?

BiP
 

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