The CMA Evolution Strategy: A Tutorial

The CMA Evolution Strategy: A Tutorial

10 Mar 2023 | Nikolaus Hansen
The CMA Evolution Strategy (CMA-ES) is a stochastic optimization algorithm designed for real-parameter optimization of non-linear, non-convex functions. It adapts the covariance matrix of the search distribution to efficiently navigate the search space. The algorithm uses a multivariate normal distribution as the search distribution, with parameters updated based on the performance of sampled points. The key components of CMA-ES include the mean update, covariance matrix adaptation, and step-size control. The mean of the search distribution is updated using a weighted average of selected points from the current generation. The covariance matrix is adapted using rank-μ and rank-one updates, which adjust the matrix based on the selected points and their evolution path. The rank-μ update uses a combination of the current covariance matrix and the covariance matrix estimated from the selected points, while the rank-one update uses the evolution path to adjust the covariance matrix. The step-size control adjusts the standard deviation of the search distribution based on the performance of the selected points. This ensures that the algorithm adapts to the problem's characteristics and maintains a balance between exploration and exploitation. The CMA-ES algorithm is robust in rugged search landscapes and can handle ill-conditioned and non-separable problems. It is particularly effective in high-dimensional spaces and for problems with complex objective functions. The algorithm's ability to adapt the covariance matrix allows it to efficiently search the space and converge to optimal solutions. The CMA-ES algorithm is implemented with careful consideration of computational efficiency and numerical stability. It uses a variety of techniques to ensure that the covariance matrix remains positive definite and that the search distribution remains well-behaved. The algorithm is also designed to handle constraints and flat fitness landscapes, making it a versatile tool for a wide range of optimization problems.The CMA Evolution Strategy (CMA-ES) is a stochastic optimization algorithm designed for real-parameter optimization of non-linear, non-convex functions. It adapts the covariance matrix of the search distribution to efficiently navigate the search space. The algorithm uses a multivariate normal distribution as the search distribution, with parameters updated based on the performance of sampled points. The key components of CMA-ES include the mean update, covariance matrix adaptation, and step-size control. The mean of the search distribution is updated using a weighted average of selected points from the current generation. The covariance matrix is adapted using rank-μ and rank-one updates, which adjust the matrix based on the selected points and their evolution path. The rank-μ update uses a combination of the current covariance matrix and the covariance matrix estimated from the selected points, while the rank-one update uses the evolution path to adjust the covariance matrix. The step-size control adjusts the standard deviation of the search distribution based on the performance of the selected points. This ensures that the algorithm adapts to the problem's characteristics and maintains a balance between exploration and exploitation. The CMA-ES algorithm is robust in rugged search landscapes and can handle ill-conditioned and non-separable problems. It is particularly effective in high-dimensional spaces and for problems with complex objective functions. The algorithm's ability to adapt the covariance matrix allows it to efficiently search the space and converge to optimal solutions. The CMA-ES algorithm is implemented with careful consideration of computational efficiency and numerical stability. It uses a variety of techniques to ensure that the covariance matrix remains positive definite and that the search distribution remains well-behaved. The algorithm is also designed to handle constraints and flat fitness landscapes, making it a versatile tool for a wide range of optimization problems.
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