What do these submaxima represent - Interference Pattern

[vanhees71]

In my notation $I_0$ is the intensity at $x=0$. Just expand around $x=0$, which gives
$$I(x) = I_0 + \mathcal{O}(x^2).$$
That's why I've put the $1/N^2$ factor in my formula.

See also the figures on p. 161 of my manuscript, cited already above.