How to calculate a negative feedback loop? Perhaps this could best be explained as a closed system between two people:
1) For every $1 person A receives, he will give 50% to person B and keep the rest.
2) For every 1$ person B receives, he will give 25% to person A, and keep the rest.
3) Now, person C hands person A $1.  How do I calculate how much money person A and person B will end up with after they keep circularly giving each other a cut of the money they just received to seemingly infinity?
An excel document I made looks like this, where each line is a step in the cycle:
Given to A  | A's Total  | Given to B | B's Total
1             1            0            0
0             0.50         0.50         0.50
0.125         0.625        0            0.375
0             0.5625       0.0625        0.4375

And so on, until after 14 cycles the differences in totals between cycles diminish and we're left with A's total of 0.571429 and B's total of 0.428571
I can solve this problem with an Excel spreadsheet, but I assume there is a formula to for this sort of feedback problem. 
One of my biggest problems finding a solution is that I don't know the correct terminology to describe the problem. 

This problem can be solved in a lot of ways but one cool way is through Markov chains. Consider the Markov chain with two states $${A, B}$$ where the probability for each state is the chance that any unit of money is in the possession of person A or B. We can define the transition matrix as 
$$ P = \begin{bmatrix}
.5 & .5 \\  
.25 & .75
\end{bmatrix} $$ where the first row, second column corresponds to the rate money moves from A to B and the and the second row, first column corresponds to the rate money moves from B to A.
The initial money distribution is $$ M_0 = [1, 0] $$ and the limit as P goes to inf is 
$$ \lim_{n \to \inf} P^n = \begin{bmatrix}
1/3 & 2/3 \\  
1/3 & 2/3
\end{bmatrix} $$
so $$ \lim_{n \to \inf} M_n = M_0 * P_n = [1/3, 2/3] $$
so A ends up with \$0.333 and B ends up with \$0.66. 
