The command intertext is used for a short interjection of one or
two lines of text in the middle of a
display alignment. Its salient feature is preservation of the alignment,
which would not happen if you simply ended the display and then started
it up again afterwards. intertext may only appear right after a
or
* command. Notice the position of the word ``and'' in
this example.
\begin{align}
A_1&=N_0(\lambda;\Omega')-\phi(\lambda;\Omega'),\\
A_2&=\phi(\lambda;\Omega')-\phi(\lambda;\Omega),\\
\intertext{and}
A_3&=\mathcal{N}(\lambda;\omega).
\end{align}