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Calculating IRR of a perpetuity with numpy / python

I am using numpy library for doing simple IRR calculations using the irr function. So for example, if I want to find the IRR of a cash flow, I do the following 

In [16]:
   import numpy as np 
   cf = np.array([-100,10,10,100])
   print np.irr(a)

0.0688601791248

But, I am looking for a way to do the NPV for a perpetuity. My cash flow would be -100, 10....

Has anyone done this before

Solution

  • PV of a perpetuity is simple:

    PV=C/i

Where C = amount of cash flows
      i = effective periodic interest rate or rate of return