@article{10_4064_cm7140s_3_2017,
     author = {Graham Hall},
     title = {Some general, algebraic remarks on tensor classification, the group $O(2,2)$ and sectional curvature in 4-dimensional manifolds of neutral signature},
     journal = {Colloquium Mathematicum},
     pages = {63--86},
     publisher = {mathdoc},
     volume = {150},
     number = {1},
     year = {2017},
     doi = {10.4064/cm7140s-3-2017},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/cm7140s-3-2017/}
}