Question

let \\(\mathbf{x} = \mathbf{x}(t)\\) be the response of the linear dynamical system
\\
\

$$\begin{aligned} x &= \\alpha x - \\beta y \\\\ y &= \\beta x + \\alpha y \\end{aligned}$$

\\
that satisfies the initial condition \\(\mathbf{x}(0) = \mathbf{x}_0\\). determine conditions on the real constants \\(\alpha\\) and \\(\beta\\) that will ensure \\(\lim_{t \to \infty} \mathbf{x}(t) = (0, 0)\\). (enter your answer as a comma-separated list of equations and inequalities.)

can \\((0, 0)\\) be a node or saddle point?

• it cannot be a node or a saddle point.
• it can be a saddle point but not a node.
• it can be a node or a saddle point.
• there is not enough information to decide.
• it can be a node but not a saddle point.

Explanation:

Analyze the system matrix and eigenvalues

Using the Linear Systems of ODEs knowledge point, we represent the system in matrix form:

$$ \mathbf{X}' = A\mathbf{X}, \quad \text{where } A =
LATEXBLOCK0
$$

The characteristic equation is:

$$ \det(A - \lambda I) = (\alpha - \lambda)^2 + \beta^2 = 0 $$

Solving for the eigenvalues \(\lambda\):

$$ \lambda = \alpha \pm i\beta $$

Determine asymptotic stability conditions

Using the Asymptotic Behavior of ODEs knowledge point, the condition for \(\lim_{t \to \infty} \mathbf{X}(t) = (0,0)\) for any initial condition \(\mathbf{X}_0\) is that the real part of all eigenvalues must be strictly negative:

$$ \text{Re}(\lambda) < 0 \implies \alpha < 0 $$

Additionally, for the system to be well-defined with distinct complex eigenvalues or real eigenvalues, we must consider the role of \(\beta\). If \(\beta
eq 0\), the eigenvalues are complex conjugates with a non-zero imaginary part, representing a spiral sink. If \(\beta = 0\), the system has a repeated real eigenvalue \(\lambda = \alpha < 0\), representing a proper node (star node). In both cases, the limit is \((0,0)\). Thus, the only condition required to ensure the limit is \((0,0)\) is:

$$ \alpha < 0 $$

Classify the critical point stability types

Using the Stability of Critical Points knowledge point, we analyze whether the origin \((0,0)\) can be a node or a saddle point under the condition \(\alpha < 0\):

• A node requires real eigenvalues of the same sign. If \(\beta = 0\), the eigenvalues are \(\lambda_1 = \lambda_2 = \alpha < 0\), which is a stable node (specifically, a proper node or star node). Thus, it can be a node.
• A saddle point requires real eigenvalues of opposite signs (\(\lambda_1 < 0 < \lambda_2\)). Here, the eigenvalues are \(\alpha \pm i\beta\). If they are real (which requires \(\beta = 0\)), they are both equal to \(\alpha < 0\). They can never have opposite signs because they are identical. Thus, it cannot be a saddle point.
• Therefore, the critical point \((0,0)\) can be a node (when \(\beta = 0\)) but not a saddle point.

Answer:

Question 1

Determine conditions on the real constants \(\alpha\) and \(\beta\) that will ensure \(\lim_{t \to \infty} \mathbf{X}(t) = (0,0)\):
<blank>\(\alpha < 0\)</blank>

Question 2

Can \((0,0)\) be a node or saddle point?

• It cannot be a node or a saddle point.
• It can be a saddle point but not a node.
• It can be a node or a saddle point.
• There is not enough information to decide.
• It can be a node but not a saddle point. (Correct answer)