Expectation value
[Expectation](https://texonom.com/expectation-e1e2f11d2f864805a5cb8a673bbfb774) 


$$E[X] = \Sigma_x{xP(X= x)} = \int{xf(x)dx}$$
### k-th moment of X
defined as the expected value of x raised to the power of k
$$E[X^k] =\Sigma_xx^kP(X = x)$$

### If only X, Y is independence, $E[XY] = E[X]E[Y]$

$E[X]=0$ does not means $E[X^2] = 0$







