diff function bug

[John Dickson]

Hi,

I think there may be a bug in the diff function.

The function returns an error when trying to differentiate a power which has a negative denominator.

For example 

For a variable name, test with value: expression("x^(1/-3)")

and then a variable name, diff_test with value: diff(test,"x")

I am getting the error message: Error evaluating variable diff_test: Don't know how to differentiate x^(1/(-3))

Not a huge issue but thought I'd flag it up.

John

[Christian Lawson-Perfect]

Thanks for reporting this

I've made the differentiation routine run the expression through the basic simplification rules first, to deal with cases like this. The rule for differentiating a power looks for plus-or-minus a rational number as the exponent, and (1/(-3)) doesn't match that. After rewriting it to -(1/3), it does match and the differentiation proceeds.

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