1. Algebra
U = ∑ a(t) Y(t)
E{U} = c Y ∑ a(t)
cov{U,V} = ∑ a(s) b(t) c YY(s-t)
U is gaussian if {Y(t)} gaussian

2. Some useful stochastic models
Purely random / white noise (i.i.d.)
(often mean assumed 0)
cYY(u) = cov(Y(t+u),Y(t)} = σY2 if u = 0
= 0 if u ≠ 0
ρYY(u) = 1, u=0
= 0, u ≠ 0
A building block

3. Random walk
Y(t) = Y(t-1) + Z(t), Y(0) = 0
Y(t) = ∑i=1t Z(i)
E{Y(t)} = t μZ
var{Y(t)} = t σZ2
Not stationary, but
∆Y(t) = Y(t) – Y(t-1) = Z(t)

4. Moving average, MA(q)
Y(t) = β(0)Z(t) + β(1)Z(t-1) +…+ β(q)Z(t-q)
If E{Z(t)} = 0, E{Y(t)} = 0
c(u) = 0, u > q
= σZ2 ∑ t=0q-k β(t) β(t+u) u=0,1,…,q
= c(-u) stationary
MA(1). ρ(u) = 1 u = 0
= β(1)/(1+ β(1) 2), k = ±1
= 0 otherwise

5. Backward shift operator remember translation operator TuY(t)=Y(t+u)
BjY(t) = Y(t-j)
Linear process.
Need convergence condition, e.g. |i | or |i |2 < 

6. autoregressive process, AR(p)
first-order, AR(1) Markov
(**)
Linear process invertible
For convergence in probability/stationarity

7. a.c.f. of ar(1) from previous slide (**)
p.a.c.f. using normal or linear definitions
corr{Y(t),Y(t-m)|Y(t-1),...,Y(t-m+1)}
= 0 for m  p when Y is AR(p)
Proof. via multiple regression

8. In general case,
Useful for prediction

9. Yule-Walker equations for AR(p).
Sometimes used for estimation
Correlate, with Xt-k , each side of

10. ARMA(p,q)
(B)Yt = (B)Zt

11. ARIMA(p,d,q).
Xt = Xt - Xt 2Xt = Xt - 2Xt-1 + Xt-2
arima.mle() fits by mle assuming Gaussian noise

12. Armax.
(B)Yt = β(B)Xt + (B)Zt
arima.mle(…,xreg,…)
State space.
st = Ft(st-1 , zt )
Yt = Ht(st , Zt )
could enclude X

13. Next i.i.d. → mixing stationary process
Mixing has a variety of definitions
e.g. normal case, ∑ |cYY(u)| < ∞, e.g.Cryer and Chan (2008)
CLT
Y-bar = ∑ t=1T Y(t)/T
Normal with
E{Y-bar} = cY
var{Y-bar} = ∑ s=1T ∑ t=1T c YY(s-t)
≈ T ∑ u c YY(u) = T σYY if white noise

14. Cumulants.
cum(Y1,Y2, ...,Yk )
Extends mean, variance, covariance
cum(Y) = E{Y}
cum(Y,Y) = Var{Y}
cum(X,Y) = Cov(X,Y)
DRB (1975)

16. Proof of ordinary CLT.
ST = Y(1) + … + Y(T)
cumk(ST) = T κ k additivity and imdependence
cumk(ST/√T) = T–k/2 cumk(ST)
= O( T T–k/2 )
→ 0 for k > 2 as T → ∞
normal cumulants of order > 2 are 0
normal is determined by its moments
(ST - Tμ)/√ T tends in distribution to N(0,σ2)