Fig2.7 <- function()
{
#  postscript(file = "Fig2.7.ps", horizontal = F, width = 6.5, height = 6.5)
  par(mfrow = c(2, 2), mar = c(1.5, 1.5, 1.5, 0.5), mgp = c(5, 0.5, 0))
  x <- seq(-0.5, 0.5, length = 1000)
  plot(x, dir(x, J = 4), type = "l")
  mtext(side = 3, line = 0.1, "J=4")
  plot(x, dir(x, J = 6), type = "l")
  mtext(side = 3, line = 0.1, "J=6")
  plot(x, dir(x, J = 8), type = "l")
  mtext(side = 3, line = 0.1, "J=8")
  plot(x, dir(x, J = 10), type = "l")
  mtext(side = 3, line = 0.1, "J=10")
#  graphics.off()
  NULL
}

dir <- function(x,J=2)
{
  sin((J + 0.5) * pi * x)/(2 * sin((x * pi)/2))
}