Let x1; x2 xn be a sample of _21 independent random variables. By taking into account that the fourth moment of a standard normal distribution is E(x4) = 3, calculate the variance of the random...

Let x1; x2 xn be a sample of _21

independent random variables. By

taking into account that the fourth moment of a standard normal distribution

is E(x4) = 3, calculate the variance of the random variable y = x1 + x2 + xn.

Suppose that the model yt = _0 +_1t+_t with t = 1n, E(_t) = 0,

(a) Find the expected value of the time series wt.

(b) Calculate Cov(wt+k;wt) and show that this covariance does not depend

on t. Is the sequence fwtg stationary?