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phase plane

The plane whose axes are a system's two unknown functions, used to plot trajectories instead of separate graphs against t.

The phase plane for a 2-D system x'=P(x,y),\ y'=Q(x,y) has x and y (the two unknown functions' values) as its axes, rather than t -- a solution's trajectory there shows the two functions' relationship, with time only implicit in the direction of travel.

See also

trajectory
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