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https://issues.apache.org/jira/browse/MATH-1176?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14246559#comment-14246559
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Thomas Neidhart commented on MATH-1176:
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I just want to add that our result is not wrong, just surprising that it
results in a diagonal element to be exactly zero, while other implementations
do return a very small number. The implementation of jama and ours are very
similar, the main difference I have seen so far was the calculation of the
squared norm.
> QRDecomposition does not detect the matrix singularity
> ------------------------------------------------------
>
> Key: MATH-1176
> URL: https://issues.apache.org/jira/browse/MATH-1176
> Project: Commons Math
> Issue Type: Bug
> Affects Versions: 3.3
> Reporter: alberto trivellato
> Priority: Minor
> Fix For: 3.4
>
> Attachments: MATH-1176.failing.patch
>
>
> QRDecomposition fails this test. The default contructor sets the threshold=0,
> so we will never have abs(Rii) <= 0
> public void testSimpleRankDeficient() throws Exception {
> double[][] A = new double[][] {
> { 1, 2, 3 },
> { 4, 5, 6 },
> { 7, 8, 9 }};
> //this matrix is singular
>
> RealMatrix M2 = MatrixUtils.createRealMatrix(A);
> QRDecomposition qr2 = new QRDecomposition(M2);
> assertFalse(qr2.getSolver().isNonSingular());//this fails
> }
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