function [ sigma ] = Sigma( Z, cluster_idx )
    pca=size(Z, 2); % optimisation PCA
    k=max(cluster_idx); % nombre de clusters
    
    sigma=zeros(k,pca);
    for i=1:k
        ind=find(cluster_idx==i); %Recherche index de tous les clusters
        V=[];
        for j=1:size(ind)
            V=[V, pca(:,j)];
        end
        sigma(i,:)=var(V');
    end
end

