hello world. Math jax. $$\sum_{n=1}^\infty {1\over n^2} = {\pi^2\over 6}$$ Here's a pic: End of LaTex stuff. $$d_{\mathbf{a}}f(\mathbf{h})=\left(\frac{\partial f}{\partial x_1}(\mathbf{a})...\frac{\partial f}{\partial x_n}(\mathbf{a})\right)\begin{pmatrix} h_1\\h_2\\\vdots\\h_n \end{pmatrix}$$