Given a multi index $\alpha = (\alpha_1,\alpha_2,...,\alpha_d)$, the order $|\alpha|$ of $\alpha$ is defined by
$$|\alpha|=\sum_{i=1}^d\alpha_i$$
Let $D^{\alpha}u = \prod_{i=1}^{d}(\frac{\delta}{\delta{x_i}})^{\alpha_i}u$ which denotes the classical partial derivative.
I couldn't figure out that what will be all the terms in the following sum:
$$\sum_{|\alpha|=2}D^{\alpha}u$$