What is the A in the Accumulation and Amount functions for the value of investments? A number of interest functions, such as the accumulation function and the amount functions include an A (sometimes capitalized, sometimes not).  Could someone tell me what the A represents?  Here's an example:
Usually one can think of $a(t)$ as some form of interest. 

So, we can normalize some initial investment to 1 so that:
$$a(0) = 1 $$

letting $k$ denote some initial investment, we can have that the value of $k$ at time $t$ is give by your formula: 

$$A(t)= ka(t)$$ 

So, for example, if you invest k=10, assume an interest rate $i=5$% and compounding interest, then ten periods after you invest your money, $k$ is worth: 

$$A(10,k) = 10*(1+.05)^{10}$$