# riem.median: Fréchet Median and Variation In Riemann: Learning with Data on Riemannian Manifolds

## Description

Given N observations X_1, X_2, …, X_N \in \mathcal{M}, compute Fréchet median and variation with respect to the geometry by minimizing

\textrm{min}_x ∑_{n=1}^N w_n ρ (x, x_n),\quad x\in\mathcal{M}

where ρ (x, y) is a distance for two points x,y\in\mathcal{M}. If non-uniform weights are given, normalized version of the mean is computed and if weight=NULL, it automatically sets equal weights for all observations.

## Usage

 1 2 3 4 5 6 riem.median( riemobj, weight = NULL, geometry = c("intrinsic", "extrinsic"), ... ) 

## Arguments

 riemobj a S3 "riemdata" class for N manifold-valued data. weight weight of observations; if NULL it assumes equal weights, or a nonnegative length-N vector that sums to 1 should be given. geometry (case-insensitive) name of geometry; either geodesic ("intrinsic") or embedded ("extrinsic") geometry. ... extra parameters including maxitermaximum number of iterations to be run (default:50). epstolerance level for stopping criterion (default: 1e-5).

## Value

a named list containing

median

a median matrix on \mathcal{M}.

variation

sum of (weighted) distances.

## Examples

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 #------------------------------------------------------------------- # Example on Sphere : points near (0,1) on S^1 in R^2 #------------------------------------------------------------------- ## GENERATE DATA ndata = 50 mydat = array(0,c(ndata,2)) for (i in 1:ndata){ tgt = c(stats::rnorm(1, sd=2), 1) mydat[i,] = tgt/sqrt(sum(tgt^2)) } myriem = wrap.sphere(mydat) ## COMPUTE TWO MEANS med.int = as.vector(riem.median(myriem, geometry="intrinsic")$median) med.ext = as.vector(riem.median(myriem, geometry="extrinsic")$median) ## VISUALIZE opar <- par(no.readonly=TRUE) plot(mydat[,1], mydat[,2], pch=19, xlim=c(-1.1,1.1), ylim=c(0,1.1), main="BLUE-extrinsic vs RED-intrinsic") arrows(x0=0,y0=0,x1=med.int[1],y1=med.int[2],col="red") arrows(x0=0,y0=0,x1=med.ext[1],y1=med.ext[2],col="blue") par(opar) 

Riemann documentation built on June 20, 2021, 5:07 p.m.