I write there the work-around I have just found in order to properly write the prime symbol in LibreOffice Math editor.
The problem is well described here OpenOffice forum topic. How to add the correct prime symbol as a special character is explained in the LibreOffice Math Guide, available at Wiki Document Foundation. Please note the difference between the prime symbol and the quotation mark. [Wikipedia article about the prime symbol][3]
Well, adding the prime symbol does not fix the issue, since that symbol takes as many space as a normal character. In the thread on the OpenOffice forum, it was suggested to use the binom command. That is the good idea. But it has to be used in parallel with nospace and size /2.
At the end, here is the workaround, waiting for LibreOffice developers to propose something shorter..
nospace{u size /2 binom{%dprime}{i}}
*Here, my purpose was to write the Favre Averaged Navier Stokes equations
