# What is correct spacing after $\forall x:$ in math formulas?

I am writing math book and I am interested how I should print them correctly. I know that the main thing is sameness across the whole document, but I am really interested in ways which are recommended by respectable persons and societies, i.e. Knuth or AMS.

Now I am editing my book. I come here with the question which spacing is correct between the following variants:

\begin{align}

• Your title question asks about the space after $\forall x:$, but in your examples you have $\forall n \in \bbN$ and the same space ($\quad$) after each of them. – ShreevatsaR Sep 21 '17 at 15:50