Generalizing Rock, Paper, Scissors game

game theory

Problem of playing rock-paper-scissors with $n$ elements and keeping it balanced

How to extend the rock-paper-scissors game to more than just $3$
elemetns, and keep it balanced?

In a balanced game, optimal strategy would be to pick every choice
with equal probability.

Is there a general rule that can be used to efficiently evaluate a winner between any two distinct options?

For example, there is one generalization to five elements, rock-paper-scissors-lizard-spock:

enter image description here

Best Answer

Stating the problem

Let $G(n)$ be a rock-paper-scissors game with $n\in\mathbb N$ elements.

Let those $n$ elements be the fist $n$ nonnegative integers $\{0,1,2,\dots,n-1\}=N_n$.

For every $n_0,n_1\in N$, if $n_0$ beats $n_1$, then $n_0$ is a "counter" to $n_1$, and $n_1$ is a "anticounter" to $n_0$. Counters and anticounters of $n_0$ are together called "interactions" of $n_0$.

We want to establish a rule for counters and anticounters of every $n_0\in N_n$, such that:

  • $n_0$ has an equal number of counters and anticounters - interactions are balanced.

  • $n_0$'s interactions in $G(n)$ should be kept in $G(n+1)$.

Solution to the problem

The first property requires $n=2k+1,k\in\mathbb N$ to be odd.

If we start observing first few cases of $G(n)$, you'll notice that to satisfy the second property, $n_0=m$ needs to be a counter to $m+1,m-2,m+3,m-4,\dots$

This is finally equivalent to the following rule: The winner between $n_0,n_1\in N_n$ is:

  • if $n_0,n_1$ have the same parity (both odd or both even), then winner is: $\max\{n_0,n_1\}$

  • else, if $n_0,n_1$ have $\ne$ parity, then the winner is: $\min\{n_0,n_1\}$

This rule works for defining counters and anticounters for every element.

This rule allows a balanced rock-paper-scissors to be played with any number of odd elements, and to easily add/remove interactions to extend or reduce the game by $2$ elements.

For example, $n=3,5$ cases and with the classic labels to elements $0,1,2,3,4$ :

enter image description here


Generalization to countable sets

Notice that the general observed rule does not depend on $n$.

Thus if we let $n\to\infty$, we can play rock-paper-scissors on $\mathbb N_0 = \mathbb N \cup \{0\}$.

We can also include negative numbers and extend it to the whole integers $\mathbb Z$, as the "parity" and "$\lt$" are still defined on integers.

We can also play it on any countable set. We establish a bijection from some countable set $X$, such as rationals $X=\mathbb Q$ for example, to either $\mathbb N_0$ or $\mathbb Z$, and call it $B(x)$.

Then the winner between $x_1,x_2\in X$ is:

$$=\begin{cases} \displaystyle \max\{B(x_1), B(x_2)\}, & B(x_1)\bmod2=B(x_2)\bmod2\\ \displaystyle \min\{B(x_1), B(x_2)\}, & B(x_1)\bmod2\ne B(x_2)\bmod2 \end{cases}$$

I believe this rule is the most "balanced" way to extend rock-paper-scissors to any finite and countable sets.

Additionally, by symmetry, we can invert the rule and keep all the properties. That is, invert the conditions for min and max calculations.

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