Math symbol examples: ${\big Delta}, \Theta, \Lambda, \Xi, \Pi, \Sigma, \Upsilon, \Phi, \Psi, \Omega$

Inline math (math in lines of text): $\mathbf{X}[k+1]=A\mathbf {X}[k]+B\mathbf{u}[k]$

Some text here and some inline math $ \left(\, \sum_{k=1}^n a_k b_k \right)^{\!\!2} \le \left(\, \sum_{k=1}^n a_k^2 \right) \left(\, \sum_{k=1}^n b_k^2 \right)$

Diplayed math (centered without surrounding text on the line): $$\mathbf{X}[k+1]=A\mathbf {X}[k]+B\mathbf{u}[k]$$ \[ x\left( x-1\right) =x^{2}-x \]

Some text here and some diplayed math $$ \left(\, \sum_{k=1}^n a_k b_k \right)^{\!\!2} \le \left(\, \sum_{k=1}^n a_k^2 \right) \left(\, \sum_{k=1}^n b_k^2 \right)$$

Aligned equations: \begin{eqnarray} mA &=&nA^{2} \label{LemmaEquationArray} \\ mA-nA^{2} &=&0 \\ A\left( m-nA\right) &=&0 \end{eqnarray}

Dollar sign \$ is $\backslash$\$ (backslash symbol followed by dollarsign with no spaces between).
Backslash $\backslash$ is \$ $\backslash$backslash\$ (dollarsign then backslash symbol then word backslash then dollarsign, all with no spaces between).