Yep, if anything the Restricted 3-Body Problem (R3BP) should be considered the "chaos theory equation", since it was Poincaré study of chaotic solutions in the R3BP that led to the discovery of sensitivity to initial conditions. Smale's horseshoe map would be another apt equation, since it succinctly describes the dynamics of chaotic systems, through the twisting and warping of the map. The Lorenz attractor is another important example that highlights the concept of deterministic chaos.