Question
Analysis. A dice problem Here is an oft-posed question. Can you construct a pair of dice other than the usual ones with 1, 2, 3,
Analysis. A dice problem
"Here is an oft-posed question. Can you construct a pair of dice other than the usual ones with 1, 2, 3, 4, 5, 6 on the faces so that the resultant probabilities are the same? First, we construct the usual addition chart and its frequency chart."
+ | 1 | 2 | 3 | 4 | 5 | 6 |
1 | 2 | 3 | 4 | 5 | 6 | 7 |
2 | 3 | 4 | 5 | 6 | 7 | 8 |
3 | 4 | 5 | 6 | 7 | 8 | 9 |
4 | 5 | 6 | 7 | 8 | 9 | 10 |
5 | 6 | 7 | 8 | 9 | 10 | 11 |
6 | 7 | 8 | 9 | 10 | 11 | 12 |
sum | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
freq | 1 | 2 | 3 | 4 | 5 | 6 | 5 | 5 | 3 | 2 | 1 |
"The entry 6, for example, indicates that there are 6 ways to achieve a total of 7.
Now suppose you have two die with the following face numbers: 1, 2, 2, 3, 3, 4 on one die and 1, 3, 4, 5, 6, 8 on the other die. As before we will construct the addition table and its frequency chart."
+ | 1 | 3 | 4 | 5 | 6 | 8 |
1 | 2 | 4 | 5 | 6 | 7 | 9 |
2 | 3 | 5 | 6 | 7 | 8 | 10 |
2 | 3 | 5 | 6 | 7 | 8 | 10 |
3 | 4 | 6 | 7 | 8 | 9 | 11 |
3 | 4 | 6 | 7 | 8 | 9 | 11 |
4 | 5 | 7 | 8 | 9 | 10 | 12 |
sum | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
freq | 1 | 2 | 3 | 4 | 1 |
"The reader should complete this frequency chart to discover a surprising result.
Why did this happen? Will this happen with other dice configurations? Let's look at the underlying algebra."
"The generating function x1 + x2 + x3 + x4 + x5 + x6gives the outcomes of one die ; just look at the exponents. For two dice look at the product (or square):"
(x1 + x2 + x3 + x4 + x5 + x6)2 =
1x2 + 2x3 + 3x4 + 4x5 + 5x6 + 6x7 + 5x8 + 4x9 + 3x10 + 2x11 +x12.
"Those coefficients are exactly the numbers in the frequency chart for a pair of regular dice! On a craps table in Las Vegas 7 is a special outcome when tossing a pair of dice; there are 6 ways it can happen:"
(1, 6), (2, 5), (3, 4), (4, 3), (5, 2), (6, 1).
"An alternate factorization of our 12th degree polynomial could reveal a "new dice" pair. Start with
x + x2 + x3 + x4 + x5 + x6 =
x (1 + x + x2 + x3 + x4 + x5) = x () = x(1 + x3) = x (1 + x + x2)(1 + x)(1 - x + x2).
Remember this is just for one die. A "new pair" of factors must multiply to the square of this 6th degree polynomial or in other words, to"
x2(1 + x + x2)(1 + x)2(1 - x + x2)2 .
"So we have eight factors to allocate to two die, keeping in mind that the factors have to behave. Each of the two ways of factoring must have an x to maintain positive outcomes. Each factor must have a 1 + x and 1 + x + x2 so that you have 6 faces, by letting x = 1. So there are not many options. In fact, there are just two ways of allocating the remaining two factors: 1 - x + x2 and 1 - x + x2. Here they are:"
- Allocate one to each die
x (1 + x + x2)(1 + x)(1 - x + x2) = x + x2 + x3 + x4+ x5 + x6
and the same for the second die
- Allocate both to one die
x (1 + x + x2)(1 + x)( 1 - x + x2)2 = x + x3 + x4 + x5 + x6+ x8
and the second die gets
x (1 + x + x2)(1 + x) = x + 2x2 + 2x3 + x4
"The first die has 1, 3, 4, 5, 6, 8 on its faces, and the second has 1, 2, 2, 3, 3, 4 on its six faces.
This new pair of dice is called Sicherman (1978 issue of Scientific American - Martin Gardner)dice or "weird dice".
Our tables show that rolling a particular sum is the same using either pair of dice. But this is not so when rolling doubles. With the usual pair of dice there are 6 ways to roll a double: (1, 1), (2, 2), (3, 3), (4, 4), (5, 5), and (6, 6), so the probability is 6/36. The "new pair" has just 4 ways to roll a double:
(1, 1), (3, 3), (3, 3) and (4, 4). The new probability is 4/36.
Also, if you allow 0 to be on a face the following pairs are produced:
011223 and 245679 ; 233445 and 023457."
Question
Label the four faces of a pair of dice in the shape of a regular tetrahedron with the integers 1, 2, 3, 4. Make both tables and reproduce the analysisabove.
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