$k$-th moment

Returns the $k$-th moment about the mean along a named dimension of all elements present. $k$ is specified by the numerical argument of the operation, which defaults to 1 (hence the result will be 0 in that case). If the dimension is not named, then the $k$-th moment is over all elements present in the tensor. Note that missing elements are not counted.

$$\langle\Delta x^k\rangle = \frac1N\sum_i(x_i-\langle x\rangle)^k$$

Also

$$\sigma^2 = \frac{N}{N-1}\langle\Delta x^2\rangle$$