par(mfrow=c(2,2))

set.seed(123)
x1<-w<-rnorm(200)
w.ts<-ts(w)
for(t in 2:200) x1[t]=x1[t-1]+w[t]
x1.ts<-ts(x1)
(x<-cbind(w.ts,x1.ts))

set.seed(123)
x2<-w<-rnorm(200)
for(t in 2:200) x2[t]=0.2+x2[t-1]+w[t]
x2.ts<-ts(x2)
(xx<-cbind(w.ts,x1.ts,x2.ts))

set.seed(123)
time<-c(1:200)
x3<-w<-rnorm(200)
for(t in 2:200) x3[t]=0.2+0.2*time+x3[t-1]+w[t]
x3.ts<-ts(x3)
(xxx<-cbind(w.ts,x1.ts,x2.ts,x3.ts))

plot(w.ts, type="l",main="백색잡음과정")
plot(x1.ts, ylab="x2",main="절편이 없는 임의보행과정",type="l")
plot(x2.ts, ylab="x2",main="절편이 있는 임의보행과정",type="l")
plot(x3.ts, ylab="x3",main="절편 및 시간추세가 있는 임의보행과정",type="l")