Opened 3 years ago

#859 new bug

Bug in gitfan.lib : invertible minor not found properly in groupActionOnQImage

Reported by: ebriand@… Owned by: somebody
Priority: minor Milestone: 4-2-0 and higher
Component: singular-libs Version: 4-1-2
Keywords: Cc:

Description

groupActionOnQImage from gitfan.lib does not work on some inputs, for instance when the input matrix Q is

intmat Q[2][3] = 1, 1, 0, 0, 0, 1;

Then the first lines in the code of groupActionOnQImage are

matrix Qmat = transpose(matrix(Q));
matrix H = gauss_nf(Qmat)
intvec indices = pivotIndices(H);
intmat Qbasis[nrows(Q)][size(indices)]=Q[1..nrows(Q),indices];
matrix QbasisInv = inverse(Qbasis);

which produces:

// ** matrix is not invertible

Indeed you can check:

> print(H);
1,0,
0,1,
0,0
> indices;
1,2
> print(Qbasis);
     1     1
     0     0

Probably gauss_nf should be applied to Q, not to its transpose Qmat.

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