= Degenerate metric tensor
A <symmetric bilinear form> on a tangent space is degenerate when some nonzero <vector> pairs to zero with every <vector>; equivalently its matrix has zero <determinant>. At such a point there is no inverse <metric tensor>, and the usual <metric tensor> construction of a unique <Levi-Civita connection> does not apply. A coordinate expression that degenerates must be checked to distinguish a removable coordinate failure from a genuinely excluded point of the geometric domain.
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