# This code is stored in:      tden.spl

#==================================#
# t.density.plot()                                 		    #
# ------------------------                       		    #
# Input : degrees of freedom; it can be a vector.#
#           (e.g.) t.density.plot(c(2,5,7,30))           #
#           creates curves for  df = 2,5,7,and 30    #
# Output: density plot of Student t distribution.    #
# Note:  Unix users must first use the  motif()     #
#           to open a grpahics window before 	    #
#           using  this function.                              #
###################################### #

t.density.plot <- function(df)
{	
   x <- seq(-4,4,,100)

   # draw x,y axis and title
	
   plot(c(-4,4), c(0,.4), type="n", 
      xlab="x", ylab="f(x;df)",
      main="Central  t  Densities")

   for(i in 1:length(df)) {
      lty.num <- 2*i-1          # specify the line types.
      f.x <- dt(x,df[i])    # calculate density.
      lines(x, f.x, type="l",lty=lty.num)   # draw spline.
    }

   # The following lines are for legends;
	
       legend( x = rep(0.85,length(df)) , 
	  y = rep(.4,length(df)) ,
	  legend = paste(as.character(df),"df") , 
	  lty = seq(1,by=2,length=length(df)) ,
	  bty = "n")
}


#  This function can be executed as follows.
#  Windows users should delete the motif( ) 
#  command.
#
#  motif( )
#  source("tden.spl")
#  par(fin=c(7,8),cex=1.2,mex=1.5,lwd=4)
#  t.density.plot(c(1,3,5,7))