Estimate Spectral Density of a Time Series from AR Fit Description Fits an AR model to x (or uses the existing fit) and computes (and by default plots) the spectral density of the fitted model. Usage spec.ar(x, n.freq, order = NULL, plot = TRUE, na.action = na.fail, method = "yule-walker", ...) Arguments x A univariate (not yet:or multivariate) time series or...