Math Equation

Use $$ to write math equation

\[\sum_{n=1}^\infty 1/n^2 = \frac{\pi^2}{6}\] \[\textbf{A}\textbf{B} = \textbf{C}\]

Use \begin{equation} to write math equation \begin{equation} \sum_{n=1}^\infty 1/n^2 = \frac{\pi^2}{6} \end{equation}

\[\begin{align*} & \phi(x,y) = \phi \left(\sum_{i=1}^n x_ie_i, \sum_{j=1}^n y_je_j \right) = \sum_{i=1}^n \sum_{j=1}^n x_i y_j \phi(e_i, e_j) = \\ & (x_1, \ldots, x_n) \left( \begin{array}{ccc} \phi(e_1, e_1) & \cdots & \phi(e_1, e_n) \\ \vdots & \ddots & \vdots \\ \phi(e_n, e_1) & \cdots & \phi(e_n, e_n) \end{array} \right) \left( \begin{array}{c} y_1 \\ \vdots \\ y_n \end{array} \right) \end{align*}\] \[f(n) = \begin{cases} n/2, & \text{if $n$ is even} \\ 3n+1, & \text{if $n$ is odd} \end{cases}\]