Which of the following processes is weakly stationary for T = f0; 1; 2; : : :g where _t is a sequence of random errors with zero-mean and variance 1 and a1 and a2 are real constants (a) _1 + _2 cos(t)...

Which of the following processes is weakly stationary for T = f0; 1; 2; : : :g

where _t is a sequence of random errors with zero-mean and variance 1 and

a1 and a2 are real constants

(a) _1 + _2 cos(t)

(b) _1 + _2 cos(t) + _3 cos(t)

(c) a1 + _1 cos(t)

(d) a1 + _1at 1 + _2