Ill-conditioning
The condition number of a matrix is the ratio of the largest singular value to the smallest singular value. A matrix is ill-conditioned if the condition number is very high, usually indicating that the lowest singular value is orders of magnitude smaller than the highest one, and rows of the matrix are heavily correlated with each other. This is a very general problem in optimization. In fact, it makes even convex optimization problems difficult to solve. Generally, neural networks have this problem, which causes SGD to get stuck, that is, learning becomes very slow in spite of the existence of a strong gradient. For the dataset with a good condition number, close to 1, the error contours are nearly circular and the negative ...
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