Test for exhaustion is definitely out of the question

True or False?

If I roll a fair standard dice any (positive) number of times and get their sum, the probability of obtaining an odd or even number is equal.

False True

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2 solutions

Jordan Cahn
Dec 3, 2018

When rolling a die once, the probability of getting an even number (1, 3, 5) is the same as the probability of getting an odd number (2, 4, 6).

Assume that when rolling n n dice, the probability of getting an even sum and the probability of getting an odd sum are both 1 2 \frac{1}{2} . Now, roll n + 1 n+1 dice.

  • An odd result occurs when the sum of the first n n dice and the final die have opposite parity. This happens with probability 1 2 × 1 2 + 1 2 × 1 2 = 1 2 \frac{1}{2}\times\frac{1}{2} + \frac{1}{2}\times\frac{1}{2} = \frac{1}{2} (you can think of this as the probability the first n n are odd times the probability the last one is even, plus the probability the first n n are even times the probability the last one is odd).

  • Similarly, an even result occurs when the sum of the first n n dice and the final die have the same parity. This also happens with probability 1 2 × 1 2 + 1 2 × 1 2 = 1 2 \frac{1}{2}\times\frac{1}{2} + \frac{1}{2}\times\frac{1}{2} = \frac{1}{2} .

The statement is True .

Considering the implied question in the problem title.

FindFit [ Table [ First [ Timing [ log 2 ( Plus@@Table [ ( Plus@@ t m o d 2 ) , { t , Tuples [ { 0 , 1 } , n ] } ] ) ] ] , { n , 25 } ] , e a x + b + c , { a , b , c } , x ] \text{FindFit}\left[\text{Table}\left[\text{First}\left[\text{Timing}\left[\log _2(\text{Plus}\text{@@}\text{Table}[(\text{Plus}\text{@@}t \bmod 2),\{t,\text{Tuples}[\{0,1\},n]\}])\right]\right],\{n,25\}\right],e^{a x+b}+c,\{a,b,c\},x\right]

a > 0.716967962881 , b > 14.4048401877 , c > 0.0089040378436 {a -> 0.716967962881, b -> -14.4048401877, c -> -0.0089040378436}

e a x + b + c /. { a 0.716968 , b 14.4048 , c 0.00890404 , x 1000 } e^{a x+b}+c\text{/.}\, \{a\to 0.716968,b\to -14.4048,c\to -0.00890404,x\to 1000\}

1.3161 × 1 0 305 1.3161\times 10^{305}

The last is the estimated time on my computer using Wolfram Mathematica in seconds for 1000 dice, which is vastly greater than the age of the universe.

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