Stochastic models - time series. Random process. an infinite collection of consistent distributions probabilities exist Random function. family of random variables, {Y(t;), t Z, } Z = {0,±1,±2,...}, a sample space
Specified if given F(y1,...,yn;t1 ,...,tn ) = Prob{Y(t1)y1,...,Y(tn )yn } n = 1,2,... F's are symmetric in the sense F(y;t) = F(y;t), a permutation F's are compatible F(y1 ,...,ym ,,...,;t1,...,tm,tm+1,...,tn} = F(y1,...,ym;t1,...,tm) m+1 t = 2,3,...
Finite dimensional distributions First-order F(y;t) = Prob{Y(t) t} Second-order F(y1,y2;t1,t2) = Prob{Y(t1) y1 and Y(t2) y2} and so on
Normal process/series. Finite dimension distributions multivariate normal Multivatiate normal. Entries linear combinations of i.i.d standard normals Y = + Z : s by 1 : s by r Y: s by 1 Z: Nr(0,I) I: r by r identity E(Y) = var(Y) = ' s by s Conditional marginals linear in Y2 when condition on it
Other methods i) Y(t;), : random variable ii) urn model iii) probability on function space iv) analytic formula Y(t) = cos(t + ) , : fixed : uniform on (-,]
There may be densities The Y(t) may be discrete, angles, proportions, vectors, ... Kolmogorov extension theorem. To specify a stochastic process give the distribution of any finite subset {Y(1),...,Y(n)} in a consistent way, in A