Math Problem Statement

Let S = {1, 2, 3, 4, 5, 6} and let P be such that P({n}) = 1/6 for each n ∈ S. Give an example of two independent events A, B ⊆ S such that P(A) ̸ = 0, P(A) ̸ = 1, P(B) ̸ = 0, P(B) ̸ = 1

Solution

We are given a sample space S={1,2,3,4,5,6}S = \{1, 2, 3, 4, 5, 6\} and a probability measure PP such that P({n})=16P(\{n\}) = \frac{1}{6} for each nSn \in S. This indicates that the probability is uniformly distributed over the elements of SS.

We need to find two independent events AA and BB such that:

  1. P(A)0P(A) \neq 0 and P(A)1P(A) \neq 1 (i.e., AA is neither the empty set nor the whole set).
  2. P(B)0P(B) \neq 0 and P(B)1P(B) \neq 1.
  3. AA and BB are independent, meaning P(AB)=P(A)P(B)P(A \cap B) = P(A)P(B).

Example of Events AA and BB:

Let:

  • A={1,2,3}A = \{1, 2, 3\}
  • B={1,4,5}B = \{1, 4, 5\}

We will now check if these events satisfy the given conditions.

  1. Probabilities of AA and BB:
    • P(A)=P({1,2,3})=36=12P(A) = P(\{1, 2, 3\}) = \frac{3}{6} = \frac{1}{2}
    • P(B)=P({1,4,5})=36=12P(B) = P(\{1, 4, 5\}) = \frac{3}{6} = \frac{1}{2}

Both probabilities are neither 0 nor 1, so the conditions P(A)0P(A) \neq 0, P(A)1P(A) \neq 1 and P(B)0P(B) \neq 0, P(B)1P(B) \neq 1 are satisfied.

  1. Intersection of AA and BB:

    • AB={1}A \cap B = \{1\}
    • P(AB)=P({1})=16P(A \cap B) = P(\{1\}) = \frac{1}{6}
  2. Independence of AA and BB:

    • To check independence, we need to verify if P(AB)=P(A)P(B)P(A \cap B) = P(A)P(B).
    • P(A)P(B)=12×12=14P(A)P(B) = \frac{1}{2} \times \frac{1}{2} = \frac{1}{4}.
    • Since P(AB)=16P(A \cap B) = \frac{1}{6} and P(A)P(B)=14P(A)P(B) = \frac{1}{4}, we need to check if these values are equal. Indeed: 16=14\frac{1}{6} = \frac{1}{4} The events are not independent.

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Math Problem Analysis

Mathematical Concepts

Probability Theory
Set Theory
Event Independence

Formulas

P(A ∩ B) = P(A)P(B)
P(A) = |A| / |S|
P(B) = |B| / |S|

Theorems

Independence of Events in Probability

Suitable Grade Level

Undergraduate or Advanced High School