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Define the cash-flow stream
The model specifies equal payments at times 1, 2, 3 and onward without a final payment date. With C = 4 and r = 8% per period, the time-zero value is 50. No fees, default changes or optional redemption terms are included.
Check why the infinite sum has a finite value
A positive rate makes the discounted terms shrink geometrically. At zero rate, positive constant payments add without a finite limit. At a negative rate, the discounted terms do not shrink in the required way. Simply substituting a nonpositive r into C/r would not value this positive infinite stream.
Account for an immediate first payment
If the first payment is at time zero with the same infinite pattern afterward, its value is the immediate C plus the ordinary C/r. Equivalently, it is C/r multiplied by 1 + r under the stated assumptions.
Do not treat every security as a level perpetuity
A preferred share or perpetual instrument can have terms that differ from a fixed payment forever. Use the simple formula for the cash flows actually assumed, and read the instrument terms for a real valuation.