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Thank you, cgnieder --once again!

Everything's working perfectly now.

Regards,
GianGesù

Thank you, cgnieder!

I have never used any newtx font. What I do is to define commands for symbols I use constantly through the document I write, so I need to load amsmath to use \DeclareMathOperator. I send you a very brief example, hoping it's a well suited MWE.

Regards,
GianGesù

Hi!

I'm a T.A. for a calculus course and I like writing the homework solutions in LyX so the students can check them out. I usually load amsmath when I write this kind of documents and I didn't have any problem until now when I tried to write an integral. The error that LyX gives is:

LaTeX Error ...