Fig7.6 <- function(phi=.5)
{
#  postscript(file = "Fig7.6.ps", height = 6.5, width = 6.5, horiz = F)
  par(mfrow = c(2, 2), mar = c(1.5, 1.5, 1.5, 0.5), mgp = c(5, 0.4, 0))
  omega <- ((1:1200) * pi)/1200
  plot(c(0, pi), c(1, 1), type = "l")
  mtext("White noise", side = 3, line = 0.1)
  plot(omega, 1/(1 + phi^2 - 2 * phi * cos(omega)), type = "l")
  mtext("AR(1), positive r", side = 3, line = 0.1)
  plot(omega, 1/(1 + phi^2 + 2 * phi * cos(omega)), type = "l")
  mtext("AR(1), negative r", side = 3, line = 0.1)
  plot(omega, 1/(1 + phi^2 - 2 * phi * cos(12 * omega)), type = "l")
  mtext("Seasonal time series", side = 3, line = 0.1)
#  graphics.off()
  NULL
}