Fréchet mean

In mathematics and statistics, the Fréchet mean is a generalization of centroids to metric spaces, giving a single representative point or central tendency for a cluster of points. It is named after Maurice Fréchet. Karcher mean is the renaming of the Riemannian Center of Mass construction developed by Karsten Grove and Hermann Karcher. On the real numbers, the arithmetic mean, median, geometric mean, and harmonic mean can all be interpreted as Fréchet means for different distance functions. Let (M, d) be a complete metric space. Let x1, x2, ..., xN be points in M. For any point p in M, define the Fréchet variance to be the sum of squared distances from p to the xi: The Karcher means are then those points, m of M, which locally minimise Ψ: If there is an m of M that globally minimises Ψ, then it is Fréchet mean. Sometimes, the xi are assigned weights wi. Then, the Fréchet variance is calculated as a weighted sum, For real numbers, the arithmetic mean is a Fréchet mean, using the usual Euclidean distance as the distance function. The median is also a Fréchet mean, if the definition of the function Ψ is generalized to the non-quadratic where , and the Euclidean distance is the distance function d. In higher-dimensional spaces, this becomes the geometric median. On the positive real numbers, the (hyperbolic) distance function can be defined. The geometric mean is the corresponding Fréchet mean. Indeed is then an isometry from the euclidean space to this "hyperbolic" space and must respect the Fréchet mean: the Fréchet mean of the is the image by of the Fréchet mean (in the Euclidean sense) of the , i.e. it must be: On the positive real numbers, the metric (distance function): can be defined. The harmonic mean is the corresponding Fréchet mean. Given a non-zero real number , the power mean can be obtained as a Fréchet mean by introducing the metric Given an invertible and continuous function , the f-mean can be defined as the Fréchet mean obtained by using the metric: This is sometimes called the generalised f-mean or quasi-arithmetic mean.