unemp <- read.table("fig.2.1.dat", header=FALSE)

plot(unemp[,1])

un <- ts(unemp[,1], start=c(1992,4), freq=4)
plot(un)

plot(diff(un))

dun1 <- diff(un)
class(dun1)

dun2 <- diff(diff(un))

plot(dun2)
lines(diff(dun2), col="red", lty=2)

ttrend <- 1:length(un)

polymod1 <- lm(un ~ ttrend)
summary(polymod1)$adj.r.squared

polymod2 <- lm(un ~ ttrend + I(ttrend^2))
summary(polymod2)$adj.r.squared

polymod3 <- lm(un ~ ttrend + I(ttrend^2)+I(ttrend^3))
summary(polymod3)$adj.r.squared

summary(polymod3)

un.hat <- ts(fitted(polymod2), start=c(1992,4), freq=4)

plot(un)
lines(un.hat, col="red", lty=2)