The present value of a single payment made at periods in the future is
(1)

where is the number of periods until payment, is the payment amount, and is the periodic discount rate. The present value of equal payments made each successive period in perpetuity (a.k.a. the present value of a perpetuity) is given by
(2)

The present value of equal payments made each successive period for periods (a.k.a. the present value of an annuity) is given by
(3)

where is the periodic payment amount.