# Katja Schiffers, Frank M. Schurr, Katja Tielbörger, 
# Carsten Urbach, Kirk Moloney, and Florian Jeltsch 



nderiv <- function(x, y)
  {
    r <- rep(NA, length(x))
       
    for(i in 2:(length(x)-3))
      {
        r[i] <- (y[i+1]-y[i-1])/(x[i+1]-x[i-1])
      }
    
    return(invisible(data.frame(x=x, dy=r)))
  }