I'm a high school teacher, and students in my probability class created the following fun conundrum. We've been stuck on it for a couple of weeks:

  • Consider this dice game played with one fair $n$-sided dice.
  • On the first turn, a roll of $n$ wins, while a roll of $1$ loses. On any other result, the player rolls again.
  • On the 2nd roll, a roll of $n$ wins, while a roll of $1$ or $2$ loses. On any other roll, the game continues.
  • On roll $k$, the player wins with a roll of $n$ and loses with a roll of $k$ or below.
  • What is the probability of winning as $n \to \infty$ ?.

The game must be won in no more than $n - 1$ turns, and for any given $n$ , $$ \mathrm{P}\left(win\right) = {1 \over n} + \sum_{i = 2}^{n - 1}\frac{\left(n - 2\right)!}{\left(n - i - 1\right)!\, n^{i}} $$

Here is where I'm stuck. Does: $$ \lim_{n \to \infty}\mathrm{P}\left(win\right) = 0? $$ or does $\mathrm{P}\left(win\right)$ converge on some other nonzero probability as $n \to \infty$ ?. How might one show this ?.

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    $\begingroup$ I think $P(win) \lt \dfrac{1}{\sqrt{n}}$. If so, it does tend to $0$ but fairly slowly $\endgroup$ – Henry Jun 13 '19 at 17:01
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    $\begingroup$ @InterstellarProbe - You are probably right, though TravisJ suggests it is not far away $\endgroup$ – Henry Jun 13 '19 at 20:30
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    $\begingroup$ @Henry - Guided by your guess and TravisJ's simulations, I was above to prove $2 / \sqrt{n}$ (see my answer). But how did you come up with the $1 / \sqrt{n}$ guess in the first place? I probably would not have succeeded without your inspired guess. $\endgroup$ – antkam Jun 13 '19 at 21:13
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    $\begingroup$ @antkam - I tried to calculate some values and spot the pattern - apparently not well enough $\endgroup$ – Henry Jun 13 '19 at 21:40
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    $\begingroup$ @antkam - trying again, I think $P(win) \times n^{n-1}$ may be OEIS A001863 and $P(win) \times (n-1)n^{n-1}$ may be OEIS A000435; the second of these is special as it started the OEIS $\endgroup$ – Henry Jun 13 '19 at 23:34

I cannot prove that $P(win) \approx {1 \over \sqrt{n}}$, but here is a proof that:

Claim: $P(win) \le {2 \over \sqrt{n}}$

which of course implies $P(win) \to 0$, answering the OP question.

Proof: For convenience, let $G =$ the OP's original game. Consider some large, fixed $n$. Define:

  • event $A =$ win $G$ on or before $\sqrt{n}$ turns

  • event $B =$ win $G$ after $\sqrt{n}$ turns

We will separately bound $P(A), P(B)$ both $\le {1 \over \sqrt{n}}$. The main claim then follows because $P(win) = P(A)+P(B)$.

Lemma: $P(A) \le {1\over \sqrt{n}}$: Imagine a modified game $G'$ where the die is always rolled for all $n$ rounds, and the game result is the first winning or losing roll. This modified game $G'$ is clearly equivalent to the OP's truncated version $G$.

Let random variable $X=$ no. of winning rolls among the initial $\sqrt{n}$ rounds of $G'$. To win on or before $\sqrt{n}$ rounds, it is necessary (though not sufficient) that $X\ge 1$, i.e. $P(A) \le P(X\ge 1)$. Meanwhile, for any non-negative integer r.v.s, we have:

$$P(X\ge 1) = \sum_{k=1}^\infty P(X=k) \le \sum_{k=1}^\infty kP(X=k) = E[X]$$

In this case, by linearity, $E[X] = {1\over n} \sqrt{n} = {1 \over \sqrt{n}}.$ Combining, we have:

$$P(A) \le P(X\ge 1) \le E[X] = {1 \over \sqrt{n}} \;\;\; \square$$

Lemma: $P(B) \le {1 \over \sqrt{n}}$: First, define:

  • event $E= $ game $G$ is inconclusive (neither won nor lost) after $\sqrt{n}$ rounds

Note that event $B$ requires event $E$, i.e. $P(B) = P(B \cap E) = P(E) P(B \mid E)$.

Now imagine two different modified games. In $G_1$, the game starts with $1$ to $\sqrt{n}$ as the losing numbers and adds one more losing number every round. By construction, $P(\text{win }G_1) = P(B \mid E)$.

In $G_2$, the game starts with $1$ to $\sqrt{n}$ as the losing numbers and the set of losing numbers doesn't change for the rest of the game. Clearly, $G_2$ is easier to win than $G_1$ (e.g. via a sample-point by sample-point dominance argument).

The modified game $G_2$ is not limited to $n$ rounds, but it does terminate with probability $1$. Therefore, the ratio:

$${P(\text{win }G_2) \over P(\text{lose }G_2)} = {P(\text{win }G_2 \mid G_2 \text{ terminates}) \over P(\text{lose }G_2 \mid G_2 \text{ terminates})} = {\text{no. of winning rolls} \over \text{no. of losing rolls}} = {1 \over \sqrt{n}}$$

The easiest proof of the above is to consider $G_2$ as a $3$-state Markov chain with $2$ absorbing states, for win and loss. Alternately one can consider there to be $1+\sqrt{n}$ terminating states, by symmetry all equally likely, and then color exactly $1$ of them as "winning" and the others as "losing".

Combining everything, we have:

$$P(B) = P(E) P(B \mid E) \le P(B \mid E) = P(\text{win }G_1) \le P(\text{win }G_2) = {1 \over 1 + \sqrt{n}} < {1 \over \sqrt{n}} \;\;\; \square$$

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This isn't a complete answer, but it suggests @Henry is approximately correct with the $P(win) \approx \frac{1}{\sqrt{n}}$.

If $p(n, k)$ is the probability of winning when $k$ is the largest "losing" value then

$$p(n,k) = \frac{1}{n} + \left(1 - \frac{k+1}{n}\right)p(n, k+1)$$

Using this easy recurrence it's straightforward to compute many values. I plotted the first 1000 values of $p(n,1)$ along with $\frac{1}{\sqrt{n}}$. They follow pretty closely (albeit not identical):

probability of win compared to 1 over the square root of n

Using python, I hit max recursion depth at around $n=2750$. At that point $p(2750,1)=0.02342415256724741$ and $\frac{1}{\sqrt{2750}}\approx 0.019069251784911846$.

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