Nothing

```
#' @title Slow Feature Analysis
#' @name sfa
#' @description
#' \code{sfa} performs Slow Feature Analysis (SFA) on a
#' \eqn{K}-dimensional time series with \eqn{T} observations.
#'
#' \strong{Important:} This implementation of SFA is just the most basic
#' version; it is merely included here for convenience in
#' \code{\link{initialize_weightvector}}. If you want to actually use full functionality of SFA in R
#' use the \pkg{rSFA} package, which has a much more advanced and efficient implementations.
#' \code{sfa()} here corresponds to \code{\link[rSFA]{sfa1}}.
#'
#' @details
#' Slow Feature Analysis (SFA) finds \emph{slow} signals (see References below). The problem has an
#' analytic solution and thus can be computed quickly using generalized eigen-value solvers.
#' For ForeCA it is important to know that SFA is equivalent to
#' finding a linear combination signal with largest lag \eqn{1} autocorrelation.
#'
#' The disadvantage of SFA for forecasting is that, e.g., white noise (WN)
#' is ranked higher than an AR(1) with negative autocorrelation coefficient
#' \eqn{\rho_1 < 0}. While it is true that WN is slower, it is not more
#' forecastable. Thus we are also interested in the fastest signal, i.e.,
#' the last eigenvector. The so obtained fastest signal corresponds to minimizing
#' the lag 1 auto-correlation (possibly \eqn{\rho_1 < 0}).
#'
#' Note though that maximizing (or minimizing) the lag \eqn{1} auto-correlation does
#' not necessarily yield the most forecastable signal (as measured
#' by \code{\link{Omega}}), but it is a good start.
#' @inheritParams common-arguments
#' @param ... additional arguments
#' @references
#' Laurenz Wiskott and Terrence J. Sejnowski (2002).
#' \dQuote{Slow Feature Analysis: Unsupervised Learning of Invariances},
#' Neural Computation 14:4, 715-770.
#' @seealso
#' \code{\link{initialize_weightvector}}
#' @return
#' An object of class \code{sfa} which inherits methods from \code{\link[stats]{princomp}}.
#' Signals are ordered from slowest to fastest.
#' @export
#' @examples
#' XX <- diff(log(EuStockMarkets[-c(1:100),])) * 100
#' plot(ts(XX))
#' ss <- sfa(XX[,1:4])
#'
#' summary(ss)
#' plot(ss)
#' plot(ts(ss$scores))
#' apply(ss$scores, 2, function(x) acf(x, plot = FALSE)$acf[2])
#' biplot(ss)
#'
sfa <- function(series, ...) {
num.series <- ncol(series)
if (is.null(num.series) || num.series == 1) {
stop("You must provide at least 2 time series (columns).")
}
out <- list(center = colMeans(series),
scale = apply(series, 2, sd),
n.obs = nrow(series),
call = match.call())
PW <- whiten(series)
whitened.series <- PW$U
Sigma.Delta.Series <- cov(diff(whitened.series, ...))
# Since U is already uncorrelated, we dont need the inverse correlation matrix of U
EE <- eigen(solve(Sigma.Delta.Series), symmetric = TRUE)
# reverse the order of the vectors and eigenvalues so that smalles (i.e., slowest) is first
loadings.tmp <- PW$whitening %*% EE$vectors
rownames(loadings.tmp) <- colnames(series)
colnames(loadings.tmp) <- paste0("SF", seq_len(num.series))
class(loadings.tmp) <- "loadings"
lambdas <- EE$values
names(lambdas) <- colnames(loadings.tmp)
out <- c(out,
list(loadings = loadings.tmp,
sdev = lambdas,
scores = whitened.series %*% EE$vectors))
colnames(out$scores) <- paste("SF", seq_len(num.series))
class(out) <- c("sfa", "princomp")
return(out)
}
```

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