An unstable node is a fixed point for which the stability matrix has both eigenvalues positive, so 
.
Unstable Node
See also
Elliptic Fixed Point, Fixed Point, Hyperbolic Fixed Point, Stable Improper Node, Stable Node, Stable Spiral Point, Stable Star, Unstable Improper Node, Unstable Spiral Point, Unstable StarExplore with Wolfram|Alpha
References
Tabor, M. "Classification of Fixed Points." §1.4.b in Chaos and Integrability in Nonlinear Dynamics: An Introduction. New York: Wiley, pp. 22-25, 1989.Referenced on Wolfram|Alpha
Unstable NodeCite this as:
Weisstein, Eric W. "Unstable Node." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/UnstableNode.html