> The original function is f(x) = M*R*(D^x), where M, R and D are constants.
> 
> I'd be using the function g(A) = sum of f(x), where x=0..A
> 
> Unfortunately, I don't know how to get a clean summation formula for this.  Even
> integration doesn't work, since I want every value from floor(x) to ceil(x) to
> return the same value; that is, the function is only defined for integral
> values of x.

Also possible worksheet function for Excel:
if we have A in B4, M in B5, R in B6 and D in B7
then the (array) summation formula can be written as

=SUM(B5*B6*B7^(ROW(INDIRECT("a1:"&"a"&B4+1))-1))

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