program logistic_map
      implicit none
      real r,x_ini,x,xi
      integer pgopen,i,i_again,isymbol

      isymbol = 1

      write (*,*) 'This program plot xn as x_n+1 = 4rx_n(1-x_n+1).'
      write (*,*)
      write (*,*) 'What is the initial value of x (0 < x < 1) ?'
      read (*,*) x_ini

      if ( pgopen('/xwin') .le. 0 ) stop
      call pgpap(5.0,0.75)
c     call pgenv(0.0, 500.0, 0.0, 1.0, 0, 0)

 100  write (*,*) 'What is the value of the control parameter r ?'
      read (*,*) r

      call pgeras
      call pgenv(0.0, 500.0, 0.0, 1.0, 0, 0)

      x = x_ini
      do i=1,500
        xi = i*1.0
        x = 4*r*x*(1-x)
        call pgpt(1,xi,x,isymbol)
      end do

 102  write (*,*) 'Again ? (1=YES;0=NO)'
      read (*,*) i_again
      if (i_again.eq.1) goto 100
      if (i_again.eq.0) stop
      goto 102

      end