Is a Shifted Solution Still Valid?

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Discussion Overview

The discussion revolves around whether a shifted solution of a homogeneous system of equations remains a valid solution. Participants explore the implications of adding a constant vector to an existing solution, focusing on the properties of linear transformations and eigenvalues.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • Some participants note that a homogeneous system implies that $A\mathbf{x} = \mathbf{0}$, questioning under what conditions $A(\mathbf{x} + \mathbf{1}) = \mathbf{0}$ would also hold.
  • One participant suggests that the shifted solution might be valid if the original solution is $(-1, -1, -1)$, leading to a trivial solution.
  • Another participant proposes simplifying the expression $A(\mathbf{x} + \mathbf{1}) = \mathbf{0}$ using the properties of linear transformations.
  • There is a discussion about whether $A$ can be zero, with some participants expressing uncertainty about this assumption.
  • It is clarified that the equation should be $A\mathbf{x} + A\mathbf{1} = \mathbf{0}$, emphasizing that $A\mathbf{1}$ represents $A$ applied to the vector $(1, 1, 1)$.
  • One participant concludes that the shifted solution is valid only if $(1, 1, 1)$ is a solution, indicating that it must be an eigenvector with eigenvalue 0, while noting that multiple eigenvectors can exist for a given eigenvalue.

Areas of Agreement / Disagreement

Participants express differing views on the conditions under which the shifted solution remains valid. While some agree on the necessity of $(1, 1, 1)$ being a solution, the discussion does not reach a consensus on the broader implications.

Contextual Notes

Participants rely on specific properties of linear transformations and eigenvalues, but the discussion does not resolve the broader implications or conditions under which the shifted solution holds.

Yankel
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Hello all

A simple question.

It is known that

\[\left ( \begin{matrix} x_{1}\\ x_{2}\\ x_{3} \end{matrix} \right )\]

is a solution of a homogenous system of equations.

I need to determine if

\[\left ( \begin{matrix} x_{1}+1\\ x_{2}+1\\ x_{3}+1 \end{matrix} \right )\]

is also a solution, and why ?

thank you !
 
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Yankel said:
Hello all

A simple question.

It is known that

\[\left ( \begin{matrix} x_{1}\\ x_{2}\\ x_{3} \end{matrix} \right )\]

is a solution of a homogenous system of equations.

I need to determine if

\[\left ( \begin{matrix} x_{1}+1\\ x_{2}+1\\ x_{3}+1 \end{matrix} \right )\]

is also a solution, and why ?

thank you !

Hi Yankel,

That depends.

A homogenous system means that $A\mathbf x=\mathbf 0$.

So when would $A(\mathbf x + \mathbf 1)=\mathbf 0$ also be a solution? (Wondering)
 
I am not sure. I will guess. Maybe when the solution is (-1,-1,-1) and then x+1 is the trivial solution ?
 
Yankel said:
I am not sure. I will guess. Maybe when the solution is (-1,-1,-1) and then x+1 is the trivial solution ?

Can you simplify:
$$\mathbf A(\mathbf x + \mathbf 1) = \mathbf 0$$
using $\mathbf A \mathbf x = \mathbf 0$ and the properties of linear transformations, which are:
$$\begin{cases}\mathbf A(\mathbf a + \mathbf b) = \mathbf A \mathbf a + \mathbf A \mathbf b \\ \mathbf A (\lambda \mathbf a) = \lambda (\mathbf A \mathbf a)\end{cases}$$
 
Oh...

A=0 ?
 
Yankel said:
Oh...

A=0 ?

Huh? No, I'm afraid not. At least not necessarily. :confused:

What I mean is if you can get rid of the parentheses in $\mathbf A(\mathbf x+ \mathbf 1) = \mathbf 0$? Then we'll see what we can deduce from that...
 
you get:

Ax+A=0

but isn't Ax=0 ?
 
Yankel said:
you get:

Ax+A=0

but isn't Ax=0 ?

$A \mathbf x$ is indeed $\mathbf 0$, but your equation should be:
$$A \mathbf x + A \mathbf 1 = \mathbf 0$$
Note that $A \mathbf 1$ actually means:
$$A \mathbf 1 = A \begin{bmatrix}1\\1\\1\end{bmatrix}$$
which is different from A.
 
this is the answer ? it's a solution only if

A1=0 ? if (1,1,1) is a solution ?
 
  • #10
Yankel said:
this is the answer ? it's a solution only if

A1=0 ? if (1,1,1) is a solution ?

Yep! (Nod)

So we can say that $\mathbf x + \mathbf 1$ will generally not be a solution, unless (1,1,1) is a solution.

I guess that should suffice to answer the question, although we can say a little more.

It means that (1,1,1) must be an eigenvector with eigenvalue 0.
And we know that $\mathbf x$ is also an eigenvector with eigenvalue 0.
However, the one does not have to be a multiple of the other, since a 3-dimensional matrix can have more than 1 eigenvector with eigenvalue 0. (Nerd)
 
  • #11
Thanks !
 

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