I am a Mahout newbie so please take this so I might be wrong, but I strongly suspect it has to do with one of your Eigenvalues being 0. That implies a singular matrix. You will see that your first two Eigenvalues are equal to the singular values. Parsing the structure in smaller eiganvals get numerically unstable in a near singular matrix. I bet that is your issue. I think you can find a description of this issue in Numerical Linear Algebra by trephethan (spelling?) and Bau.
On Nov 8, 2011, at 4:11 AM, motta <motta....@gmail.com> wrote: > Hi everybody, > I have completed my first Mahout experiment with an Hadoop local > installation (single machine) and I obtained different results from Scilab > and the Mahout Distributed Lanczos Solver. Could someone explain why this > happens? Am I doing something wrong? > > This is my matrix > 2,0,8,6,0 > 1,6,0,1,7 > 5,0,7,4,0 > 7,0,8,5,0 > 0,10,0,0,7 > > This is my Mahout invocation > ./hadoop jar > /home/hadoop-user/mahout/mahout-distribution-0.5/mahout-examples-0.5-job.jar > org.apache.mahout.math.hadoop.decomposer.DistributedLanczosSolver --input > /user/hadoop-user/mahout-input --output /user/hadoop-user/mahout-output > --numCols 5 --numRows 5 --cleansvd "true" --rank 5 > > These are the Mahout results > 11/11/08 12:45:04 INFO lanczos.LanczosSolver: 4 passes through the corpus so > far... > 11/11/08 12:45:04 INFO lanczos.LanczosSolver: Lanczos iteration complete - > now to diagonalize the tri-diagonal auxiliary matrix. > 11/11/08 12:45:04 INFO lanczos.LanczosSolver: Eigenvector 0 found with > eigenvalue 0.0 > 11/11/08 12:45:04 INFO lanczos.LanczosSolver: Eigenvector 1 found with > eigenvalue 1.0869992925693057 > 11/11/08 12:45:04 INFO lanczos.LanczosSolver: Eigenvector 2 found with > eigenvalue 3.4305998309907 > 11/11/08 12:45:04 INFO lanczos.LanczosSolver: Eigenvector 3 found with > eigenvalue 15.171371217397603 > 11/11/08 12:45:04 INFO lanczos.LanczosSolver: Eigenvector 4 found with > eigenvalue 17.918370809987454 > 11/11/08 12:45:04 INFO lanczos.LanczosSolver: LanczosSolver finished. > > And these are the results from Scilab (svd(X)) > -->[U,S,V]=svd(X); > -->S > S = > > 17.918371 0. 0. 0. 0. > 0. 15.171372 0. 0. 0. > 0. 0. 3.564002 0. 0. > 0. 0. 0. 1.9842282 0. > 0. 0. 0. 0. 0.3495557 > > thank you, > Alfredo > > > -- > View this message in context: > http://lucene.472066.n3.nabble.com/Comparing-results-of-Mahout-SVD-and-Scilab-tp3490066p3490066.html > Sent from the Mahout User List mailing list archive at Nabble.com.
