The following formula is involved in the calculation of present value interest factors for an annuity:

Present value interest factor of an annuity:
where R is the periodic interest rate and N is the number of periods.

The following references can be used to cite this present value interest factor of an annuity (pvifa) calculator:
 Soper, D.S. (2016) "Present Value Interest Factor of an Annuity (PVIFA) Calculator (Online Software)", http://www.danielsoper.com/fincalc. 
 Vance, D.E. (2005). "Raising capital", New York, NY: Springer. 