Accumulation function

In actuarial mathematics, the accumulation function a is a function of time t expressing the ratio of the value at time t and the initial investment. It is used in interest theory.
Thus a = 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 :

simple interest:
compound interest:
simple discount:
compound discount:

In the case of a positive rate of return, as in the case of interest, the accumulation function is an increasing function.

Variable rate of return

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: