The accumulation function a(t) is a function defined in terms of time t expressing the ratio of the value at time t (future value) and the initial investment (present value). It is used in interest theory.
Thus a(0)=1 and the value at time t is given by:
where the initial investment is
For various interest-accumulation protocols, the accumulation function is as follows (with i denoting the interest rate and d denoting the discount rate):
In the case of a positive rate of return, as in the case of interest, the accumulation function is an increasing function.
The logarithmic or continuously compounded return, sometimes called force of interest, is a function of time defined as follows:
which is the rate of change with time of the natural logarithm of the accumulation function.
Conversely:
reducing to
for constant .
The effective annual percentage rate at any time is: