Space:"\ " will give you a space in math expressions, notice the space after "\".

Vector and bar:  \bar a  gives you $$\bar a$$,  \vec a  gives you $$\vec a$$ and  gives you $$\overline{AB}$$.

Under and above: \underset{a\ note\ here}{x+y+z}  gives you $$\underset{a\ note\ here}{x+y+z}$$ and \overset{a\ note\ here}{x+y+z} gives you $$\overset{a\ note\ here}{x+y+z}$$. To make it looks better we can us the code \underset{a\ note\ here}{\underbrace{x+y+z}} to get

$$\underset{a\ note\ here}{\underbrace{x+y+z.}}$$

Similarly  \overset{\text{ Some texts}}{\overbrace{(a+b+c+d)^2}}  gives you

$$\overset{\text{ Some texts}}{\overbrace{(a+b+c+d)^2.}}$$

Some special characters:

For the following special characters, double-click it and then you can select and copy code by using your mouse.

$$\alpha$$, $$\beta$$, $$\gamma$$, $$\delta$$, $$\epsilon$$, $$\varepsilon$$, $$\zeta$$, $$\eta$$, $$\theta$$, $$\vartheta$$, $$\iota$$, $$\kappa$$, $$\lambda$$, $$\mu$$, $$\nu$$, $$\xi$$, $$o$$, $$\pi$$, $$\varpi$$, $$\upsilon$$, $$\phi$$, $$\varphi$$, $$\chi$$, $$\psi$$, $$\omega$$, $$\Gamma$$, $$\Delta$$, $$\Theta$$, $$\Lambda$$, $$\Xi$$, $$\Pi$$, $$\Sigma$$, $$\Upsilon$$, $$\Phi$$, $$\Psi$$, $$\Omega$$, \\$, $$\infty$$, $$\cap$$, $$\bigcap$$, $$\cup$$, $$\bigcup$$ , $$\geq$$, $$\leq$$, $$\neq$$, $$\doteq$$, $$\propto$$, $$\asymp$$, $$\ll$$, $$\gg$$, $$\equiv$$, $$\sim$$, $$\simeq$$, $$\approx$$, $$\cong$$, $$\rightarrow$$, $$\Rightarrow$$,$$\leftarrow$$, $$\Leftarrow$$, $$\leftrightarrow$$, $$\Leftrightarrow$$, $$\uparrow$$, $$\downarrow$$, $$\updownarrow$$, $$\Uparrow$$, $$\Downarrow$$, $$\Updownarrow$$, $$\cdot$$, $$\cdots$$, $$\vdots$$, $$\ddots$$, $$\angle$$, $$\emptyset$$, $$\bot$$, $$\triangle$$, $$\nabla$$, $$\partial$$, $$\heartsuit$$, $$\spadesuit$$, $$\clubsuit$$, $$\forall$$, $$\exists$$, $$\hbar$$, $$\sharp$$, $$\natural$$, $$\flat$$, $$\div$$, $$\times$$, $$\pm$$, $$\mp$$, $$\star$$, $$\circ$$, $$\bullet$$, $$\otimes$$, $$\oplus$$, $$\ominus$$, $$\oslash$$, $$\odot$$, $$\amalg$$, $$\dagger$$, $$\ddagger$$, $$\nearrow$$, $$\searrow$$, $$\nwarrow$$, $$\swarrow$$, $$\vee$$, $$\wedge$$, $$\sqcup$$, $$\bigcirc$$, $$\diamond$$,