2.2.1 Normal Equation by Andrew Ng

Computing Parameters Analytically
Machine Learning - Stanford University | Coursera
by Andrew Ng
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[@dream191919]

There is an error of the example Andrew used here to demonstrate Normal equation. The X is the 4 by 5 matrix which makes the system underdetermined, which also result in the inverse of X's transpose multiplying X having no inverse. So the Normal equation cannot be calculable.

[@bonipomei]

X is 4x5 and X(transpose) is 5x4. Therefore, X(transpose)*X = 5x4 * 4x5 which results in a 5x5 matrix, which has an inverse.