C
      FUNCTION f(x,r,iterate)
      IMPLICIT NONE
      REAL*8 f, x, y, r
      INTEGER iterate
      IF(iterate .GT. 1) THEN
         y = f(x,r,iterate - 1)
         f = 4*r*y*(1 - y)
      ELSE
         f = 4*r*x*(1 - x)
      END IF
      END