Why is there a space in this equation

Question

I have the following equation: $\|\text{DFT}\left( f_t (n) \right)\|^2 $ ()

This equation is rendered with a space before the second $\|$. Why is this and how can I fix it?

Answer 1

Use $\left\|\text{DFT}\left( f_t (n) \right)\right\|^2$. Otherwise LaTeX does not know that the first \| is supposed to be a left delimiter and the second \| a right one.

Answer 2

This is due to some weird design choices by Knuth. Notice that there's also an extra space after \text{DFT}. They are produced because \left and \right follow different spacing rules.

To obtain the expected spacing, use

$\|\text{DFT}\mathopen{}\left( f_t (n) \right)\mathclose{}\|^2$

For a more complete discussion, see this answer.