coverage: 94.504%. remained the same
when pulling 60a7bfa on pierre-haessig:doc-acker
into 87a0550 on python-control:main.

Oups, I didn't see PR #1007 and pushed my own commit to fix place_varga doctring. Now I have to fix the conflict!

Now I have to fix the conflict!

Done!

@pierre-haessig Thanks! I am going to squash-merge this because the second commit is not used after you resolved the conflict.

Regarding the other two items in your opening post:

1. For widely known results like Ackermann's formula, I recommend to use an open access reference, if available. The book Feedback Systems can be used for this. Ackermann's formula is given in equation (7.21) in Chapter 7.
2. Ackermann's formula is only for single-input systems. This aligns with the similar acker function in Matlab. The function always raises an exception if the system has more than 1 input, but the error message may be confusing. We could add a check with ValueError as you suggest, or the documentation can add a note about single-input systems (or both). Feel free to create another pull request with it.

As an aside, I was not able to find a description of acker in the official Matlab documentation. Perhaps they removed it?

As an aside, I was not able to find a description of acker in the official Matlab documentation. Perhaps they removed it?

Neither did I. I only found an online copy of a unspecified Matlab version here: www.ece.northwestern.edu/local-apps/matlabhelp/toolbox/control/ref/acker.html (in the source code, the date 2002 is mentioned). I don't know when it was removed.

[Edit: sorry, I didn't see you were mentioning the exact same link for Matlab's acker doc...]