English: A vector field ${\displaystyle ({\dot {x}},{\dot {y}})=(-y-x^{3},x^{5})}$ in the plane. The function ${\displaystyle V(x,y)=x^{6}+3y^{2}}$ satisfies ${\displaystyle {\dot {V}}=-6x^{8}}$, which satisfies the LaSalle's invariance principle, showing that the origin is asymptotically stable.

Plotted in Julia by the following code.

```julia using CairoMakie, LinearAlgebra

1. Lyapunov function example from https://web.archive.org/web/20190714024047/http://www.math.byu.edu/~grant/courses/m634/f99/lec23.pdf

df(x, y) = [-y - x^3; x^5] lyapf(x, y) = x^6 + 3y^2

using Interact

1. define Figure

scene = Figure(resolution = (1600, 3200)) Axis(scene[1,1], backgroundcolor = "black")

1. define (x, y) points for plotting

xmin, xmax, xres = -1, 1, 41 ymin, ymax, yres = -1, 1, 41

x = range(xmin, stop = xmax, length = xres) y = range(ymin, stop = ymax, length = yres) xs = repeat(x, outer=length(y)) ys = repeat(y, inner=length(x))

1. plot vector field

vectors = df.(xs, ys) us = map((x) -> x[1], vectors) vs = map((x) -> x[2], vectors) us /= 5 vs /= 5 n = vec(norm.(vectors)) n /= maximum(n) / 10 arrows!(xs, ys, us, vs, arrowsize = n, linecolor=n, arrowcolor = :white)

Axis(scene[2,1], backgroundcolor = "black")

1. plot contour of Lyapunov function for the vector field

arrows!(xs, ys, us, vs, arrowsize = n, linecolor=n, arrowcolor = :white)

1. plot contour of Lyapunov function for the vector field

zs = lyapf.(xs, ys) Makie.contour!(xs, ys, zs, levels = 30, linewidth = 2, colormap = :grayC)

1. display plot
2. scene

save("LaSalle.png", scene)

```