说明：  径向基函数是某种沿径向对称的标量函数，通常定义为样本到数据中心之间径向距离（通常是欧氏距离）的单调函数（由于距离是径向同性的）
(The radial basis function is some scalar function that is symmetric along the radial direction. It is usually defined as the monotonic function of the radial distance (usually Euclidean distance) between the sample and the data center.)