I want a single equation number located between the second and third lines (vertically centered behind the lines):
as in Section 18.6 of the LyX detailed Math manual:
In Lyx, I choose "insert" → "Math" → "numbered formula", then right-lick the written equation to choose AMS align environment, and use
\hspace and Ctrl+Space to adjust manually, but I get all lines numbered!
I try to delete the unwanted numbers by right clicking and unticking "number this line". This removed only the last number: the top three are still numbered, which is weird: