Investigation of the above problems started at the end of the 1980s with work on bifurcation control and continued with work on the control of chaos. Ott, Grebogi and Yorke and their followers introduced a new class of control goals not requiring any quantitative characteristic of the desired motion. Instead, the desired qualitative type of the limit set (attractor) was specified, e.g., control should provide the system with a chaotic attractor. Additionally, the desired degree of chaoticity may be specified by specifying the Lyapunov exponent, fractal dimension, entropy, etc. See.