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In the paper Robot Manipulability they use the symbol (???) after several equations ( (2.5), (2.6), (2.9), ...). Since I've never seen this before, I'm wondering is this should be some kind of notation for e.g. assumption? Or could it be related to the pseudo inverse they use?

Since the paper might be closed-access: Screenshot of (2.5)

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  • $\begingroup$ Probably the book is formatted with TeX and these question marks stand for cross-references that cannot be found in the book when the book is produced by the program. $\endgroup$ – Saad Dec 27 '17 at 10:45
  • $\begingroup$ it doesn't look like that, because the references are set correctly and it is not aligned right like a equation number. In addition, I guess the editor of IEEE transactions would have noticed that. The (???) always appears next to equations where the Moor-Pseudo Inverse is used, that's why I think it should be some kind of notation. $\endgroup$ – madn Dec 27 '17 at 11:56
  • $\begingroup$ I've added a screenshot for clarification. $\endgroup$ – madn Dec 27 '17 at 12:15
  • $\begingroup$ You can be sure that these question marks are there unwillingly. Either because of an unresolved reference, a self-comment of the author about something to check, or similar. $\endgroup$ – Yves Daoust Dec 27 '17 at 13:35
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The answer is: the question marks have no special meaning, they have to be there without purpose.

Since extended search plus asking a few mathematicians didn't come to any other solution.

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