# A standard geometric distribution can be interpreted as the number of Bernoulli trials required to get one success. However, what if the probability of success if each trial diminishes by some factor with each failure?

A Diminishing Geometric Distribution
A standard geometric distribution can be interpreted as the number of Bernoulli trials required to get one success. However, what if the probability of success if each trial diminishes by some factor with each failure?
Let p be probability of success of the first trial and d be the diminishing factor of each failure such that the probability of success of trial n is $p{d}^{n-1}$. I have been trying to calculate the expected value of this distribution without success. My main stumbling block is the probability of failing n times:

Given that, the expected value would be
$E\left(X\right)=\sum _{n=1}^{\mathrm{\infty }}np{d}^{n-1}F\left(n-1\right)$
Is there a way to get a closed form for this?
You can still ask an expert for help

• Questions are typically answered in as fast as 30 minutes

Solve your problem for the price of one coffee

• Math expert for every subject
• Pay only if we can solve it

Arthur Gillespie
Step 1
The random variable X is not well-defined. For any $d<1$, there is a non-zero probability that there will never be a success.
To show this, first note that it is clear that for any n, there is a non-zero probability that the game will last at least n trials. We can choose n large enough that $p{d}^{n-1}$ is as small as we wish. Given that there have been n trials without a success, the probability that there will never be a success is $\prod _{k=n+1}^{\mathrm{\infty }}\left(1-p{d}^{k-1}\right)$. For small $p{d}^{k-1}$, we can lower bound the logarithm of this. For example, it is $>-\sum _{n+1}^{\mathrm{\infty }}2p{d}^{k-1}$