Extra ticket per ticket sold I have entered a lottery with 1 ticket, and I get an extra ticket per every other ticket I sell. I don't know if there is a maximum number of tickets or how many tickets have been sold. Every person that gets a ticket gets the same option of getting an extra ticket per every other ticket sold.
What is the probability that I win?
 Does it make sense to sell more tickets or should I just stick to mine?
 A: Without knowing the exact details of your lottery, there are some general ideas that are at play here.
When you sell a ticket, you gain a ticket for yourself (good), but create another ticket that isn't yours and risk the other person creating more tickets (both bad).
Let's define the expected number of tickets that the person you are selling to will go on to create as $E$. We'll call the tickets sold so far as $N$ and consider your expected chance to win by selling $i$ tickets. We'll assume there are no ties in this lottery and that tickets have no additional cost or benefit to selling. Your chance of winning is the number of tickets you own divided by the total number of tickets. Each ticket sold by you adds 1 to the numerator and $E+2$ to the denominator: one for you, one for your buyer and $E$ that your buyer goes on to produce. Then:
$P(\textrm{win}|i) = \frac{1+i}{N+(E+2)i} $
If we take $N=1000$, $E=2$ and consider selling between 1 and 10,000 tickets, the expected chance of winning looks like this:

It clearly converges in the regime of selling $\approx N$ tickets, to $1/(E+2)$ if you want to sell infinite tickets:
$P(\textrm{win}|i=\infty) \approx \frac{1}{E+2} $
So unless you expect the person you sell to will go on to cause the creation of $N-2$ or more tickets, it's always a good idea to sell a ticket, and even better if you make money selling them.
