# add helping materials 

pv_perpetuity<-function(c,r){
"Objective: estimate the present value of perpetuity
                                 c
   Formula:   pv(perpeturity) = ------
                                  r
    c  : cash flow with the 1st one at the end of 1st period
    r  : effective period rate

 Example #1: pv_perpetuity(10,0.08)
             [1] 125   

 Example #2: pv_perpetuity(r=0.10,c=100)
            [1] 1000

"
return(c/r)
}