fishertest <- function(x,Lag.max=50)
{ 
   #x is a time series to be tested whether it is a white noise by using
   #the Fisher test 
   #Output:  The Fisher test statistic and P-value

   pgram <- Mod(fft(x))^2/length(x)/2/pi
   pgram <- pgram[-1]       #delete the zero frequency component
   pgram <- pgram[1:floor(length(x)/2)]  #take the first half
   Fisher <- max(pgram)/mean(pgram)  #Fisher statistic
   P.value <- 1-exp(-exp(-Fisher+log(length(pgram))))
   list(Fisher.stat=Fisher, P.value=P.value)
}