In this setting, e 0 = 1, and e x is invertible with inverse e −x for any x in B If xy = yx, then e x y = e x e y, but this identity can fail for noncommuting x and y Some alternative definitions lead to the same function For instance, e x can be defined as → ()The derivative of e x is e x This is one of the properties that makes the exponential function really important Now you can forget for a while the series expression for the exponential We only needed it here to prove the result above We can now apply that to calculate the derivative of other functions involving the exponential Example 1 fCalculating the derivative of x^x is a very simple task, but it may be hard to find the general idea on your own, so here it is We will need the following formula (where " \log " denotes the natural logarithm, which is often denoted " \ln " in nonmathematical literature) The trick is to
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