How to specify Align separator with & in LyX?

Question

I need to add & just before = in the aligned environment in LyX to specify the deliminator. So

How to specify the deliminator with & in the aligned environment with LyX?

Answer 1

Each of those blue boxes in a row specifies something that will be separated by an & in LaTeX. So while you currently have four columns with this content (e.g. on the first row):

• Nm(1)=Nm(1+0\sqrt{-5})
• [blank]
• [blank]
• Nm(ab)=Nm(a)Nm(b)

... you should instead replace it with four columns with the following content:

• Nm(1)
• =Nm(1+0\sqrt{-5})
• Nm(ab)
• =Nm(a)Nm(b)

If you want more equations in a row, use the add column button in the table toolbar at the bottom of the screen (it only appears when your cursor is in the equation).

Maybe this seems like LyX is making it hard to get to that LaTeX feature, but those LaTeX &s are column dividers, so if anything I'd say LyX's representation is closer to the philisophical meaning. BTW, a corollary of this is that you cannot have different numbers of &s in different rows, which seemed to be what you're suggesting. LyX's column system makes it impossible to accidentally do this.

Edit: As an unrelated comment, if Nm is meant to be a single identifier rather than the product of N with m, the convention is that it should have upright letters (as e.g. \sin and \exp do). Use a LyX math macro with \operatorname{Nm} for this; see e.g. this blog post for details.

Answer 2

As Jim Oldfield answered, the first row with $I\cdot I$ and the second column with all equality signs below. Also the option Insert > Math > Aligned environment to add the aligned environment in the first hand: I did first adding the array and then the aligned but there may be more convenient ways to this, below the graphical way.