Markus,
Thanks for sending me your question. Attached is an image I get when I run your question on my demo server https://stack-demo.maths.ed.ac.uk/demo/ which is running 2020120600. If this is what you wanted (and we certainly have negative exponents here!) but not what you get then we should exchange some more private emails about which version of Maxima you are running and try to track down what is wrong.
Thanks for sending me your question. Attached is an image I get when I run your question on my demo server https://stack-demo.maths.ed.ac.uk/demo/ which is running 2020120600. If this is what you wanted (and we certainly have negative exponents here!) but not what you get then we should exchange some more private emails about which version of Maxima you are running and try to track down what is wrong.
Chris
