I have some small issue about the Jacobian. Why do we even need to take the absolute value of the determinant? In single variable case, the derivative can similarly be thought of as the "exchange rate" for the dx to du if we change variables and in the sense similar to the Jacobian but in there even if the detivative is negative we don't take absolute values. But why should we take absolute values in the Jacobian determinant case?####[['Jacobian', 'Mathematical_Related_Concept', 'NEU'], ['absolute value', 'Mathematical_Related_Concept', 'NEU']]