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 NEF Compute Negative Eigen Fraction from PE dataset or mapping
     [F,R] = NEF(W) 
 
 
 DescriptionThe negative eigen-fraction is a ratio expressing the sum of magnitudes of all negative eigenvalues divided by the sum of magnitudes of all eigenvalues. The necessary numbers are stored in W = PE_EM(D) or in A = D*W, in which D is a symmetric dissimilarity matrix. The negative eigen-fraction is the ratio of the magnitude of the lowest negative eigenvalue and the largest positive eigenvalue. See also
 
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