Equilibria and stability

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Find all equilibria for the following system and determine their stability:

$$x'=y^2-4$$ $$y'=x^2-1$$

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We find four critical points as:

$$(x, y) = (-1,-2),(1,-2), (-1,2), (1,2)$$

We evaluate the Jacobian of the system as:

$$J(x, y) = \left( \begin{array}{cc} 0 & 2 y \\ 2 x & 0 \\ \end{array} \right)$$

We now evaluate the eigenvalues at each critical point and find two centers and two saddles.

A phase portrait with nullclines shows:

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