et
be observation time. We consider an agreement to invest at time
,
,
a fixed amount of cash and collect at
,
,
one unit of reference currency. We denote
the simple compounding rate during the time interval
implied by such a contract. We replicate this contract by selling
units of bond
and purchasing one unit of bond
.
If the implied rate
is fair then the contract should not worth anything at time
:
The cash flow at time
is
.
According to the contract, the investment will grow at the rate
up to the time
when it pays 1 unit of currency.
Therefore
We conclude that the fair rate for the contract is given by the
relationship
|
|
(Libor)
|
By definition, this is the "forward LIBOR". The structure of the last formula
is such that the rate
is a
martingale
with respect to the
forward
probability measure
Prob
Prob
.
|