diffloggdp=diff(logGDP)
Box.test(diffloggdp,lag=12,type=c("Ljung-Box"))
m1=ar(diffloggdp,method='mle')
m1
Box.test(m1$residuals,lag=12,type=c("Ljung-Box"))
residuals(m1)


Call:ar(x = diffloggdp, method = "mle")Coefficients:      1        2        3   0.3461   0.1298  -0.1224  Order selected 3  sigma^2 estimated as  8.258e-05> Box.test(m1$residuals,lag=12,type=c("Ljung-Box"))Error in ts(x) : 'ts'对象至少必需有一个或多个观察量求助各位，为什么结果是null