As I was going through the table of contents, I faced the problem of typing a formula correctly, then after eventually deleting it because it caused many kinds of errors, LaTeX cannot compile anymore because of the error in the title.
I obviously did research on the Internet yet I couldn't find an appropriate answer to my problem. For example, I do not know how to delete auxiliary files, which could be the cause. So, please help me!
And maybe you could give me an hint on how to type correctly the formula:
I did try: