Calculus, Physics.
Values are arbitrarily chosen, as given in the book. They can be tweaked to fit any ecosystem. Next we define the function required. They are as per the differential defined. a, b, c, d = 0.4, 0.002, 0.001, 0.7 def f(xy, t): x, y = xy return[a * x - b * x * y, c * x * y - d* y] Setting the initial