2

We are playing a fair tossing game. Each player chooses a sequence of 3 outcomes. Whoever's sequence is reached first wins the game. Player 1 chooses first and player 2 chooses second. There are 8 possible choices for player 1 and 7 for player 2.

Should you go first or second to have the highest probability of winning?

The brute force way of solving this problem is to enumerate all possible pairs. There are $\binom{8}{2} = 28$ unique pairs. A lot of pairs. Though I think another set of symmetry arguments could further cut this by half down to 14. For example, considering the case HHH is the same as considering the case TTT, or more generally, consider the case XYZ is the same as considering the case $\bar{X}\bar{Y}\bar{Z}$.

A smarter way to approach this problem is based on the reasoning that player 2 should always choose the first 2 outcomes of player 1's sequence as the last 2 outcomes of its sequence. How can I prove this in a relatively easy way?

For example, say, player 1 chooses HHH. Player 2 should choose THH in this case. In this case player 1 only has a $\frac{1}{8}$ of winning. If the first 3 tosses are not all heads, then player 2 is guaranteed to win because the sequence $HH$ in $THH$ will always occur before $HHH$. If a tail occurs in any of the first 3 tosses, then player 1 can never win. This case was simple for me to reason.

Now, say, player 1 chooses HTT. Based on my logic, player 2 should choose THT or HHT, but it's not clear to me which one is a better choice by inspection, unlike the previous case where we only had 1 possibility. So we have 2 choices this time. How can I quickly think about which choice is a better choice? Or in other words, how do I determine whether the first outcome in player 2's sequence should be a tail or head?

24n8
  • 1,523
  • 3
    https://en.wikipedia.org/wiki/Penney%27s_game – saulspatz May 07 '20 at 19:40
  • @saulspatz Thanks. Didn't know the name of the game. I don't think they explained why the first choice for player 2 should be the opposite of the middle choice for player 1. Their explanation at the end of the "Analysis of the three-bit game" section seems to only talk about why player 2's last 2 choices should be the first 2 choices of player 1. Can someone explain this? – 24n8 May 07 '20 at 20:00
  • @Iamanon: Have you seen this post and the links in the comments there? Also this one: https://math.stackexchange.com/questions/66670 and this one: https://math.stackexchange.com/questions/3381462. Searching the site for "Penney's game" also brings up some questions. I would tend to close this question as a duplicate of one of those – would that be OK with you? If not, please say what you're missing in the answers to those questions. – joriki May 08 '20 at 01:39
  • @joriki Thanks I'll take a look at them tomorrow. I didn't know this game had a name, so I wasn't sure of what to search for. – 24n8 May 08 '20 at 03:08
  • @Iamanon: I understand; I didn't mean to imply that you should have found them by searching without knowing the name (though I did find some of them by searching for HTT and THH :-). Normally I'd just go ahead and close this as a duplicate; I just wanted to make sure because you're looking for a way to "quickly think" about this and are already aware of a solution that you don't find satisfactory, and I wasn't sure whether any of the solutions there meet that description. – joriki May 08 '20 at 06:17
  • @joriki Yup, they do. Feel free to close it. I'm not sure how to do that. (I don't want to delete the question because I occasionally go through questions that I've asked as a refresher). – 24n8 May 08 '20 at 12:43

0 Answers0