Enter the inline formula like this: \ (dim \; \; Im\{fi\}\le n- 1\) will do.

The effect is as follows:

Where \ (... \) is the general convention for entering embedded formulas. Or you can use dollars ... $,which is the same.

\; Command is used to control spaces. You can also try \, or \: or \ (backslash with spaces) or \quad.