flip a coin 4n times. the most probable number of heads is 2n, and its probability is p(2n). if the probability of observing n heads is p(n), show that the ratio p(n)/p(2n) diminishes as n increases.

Answers

Answer 1

The most probable number of heads becomes more and more likely as the number of tosses increases.

Let's denote the probability of observing tails as q (which is 1/2 for a fair coin). Then the probability of observing exactly n heads in 4n tosses is given by the binomial distribution:

p(n) = (4n choose n) * (1/2)^(4n)

where (4n choose n) is the number of ways to choose n heads out of 4n tosses. We can express this in terms of the most probable number of heads, which is 2n:

p(n) = (4n choose n) * (1/2)^(4n) * (2^(2n))/(2^(2n))

= (4n choose 2n) * (1/4)^n * 2^(2n)

where we used the identity (4n choose n) = (4n choose 2n) * (1/4)^n * 2^(2n). This identity follows from the fact that we can choose 2n heads out of 4n tosses by first choosing n heads out of the first 2n tosses, and then choosing the remaining n heads out of the last 2n tosses.

Now we can express the ratio p(n)/p(2n) as:

p(n)/p(2n) = [(4n choose 2n) * (1/4)^n * 2^(2n)] / [(4n choose 4n) * (1/4)^(2n) * 2^(4n)]

= [(4n)! / (2n)!^2 / 2^(2n)] / [(4n)! / (4n)! / 2^(4n)]

= [(2n)! / (n!)^2] / 2^(2n)

= (2n-1)!! / (n!)^2 / 2^n

where (2n-1)!! is the double factorial of 2n-1. Note that (2n-1)!! is the product of all odd integers from 1 to 2n-1, which is always less than or equal to the product of all integers from 1 to n, which is n!. Therefore,

p(n)/p(2n) = (2n-1)!! / (n!)^2 / 2^n <= n! / (n!)^2 / 2^n = 1/(n * 2^n)

As n increases, the denominator n * 2^n grows much faster than the numerator (2n-1)!!, so the ratio p(n)/p(2n) approaches zero. This means that the probability of observing n heads relative to the most probable number of heads becomes vanishingly small as n increases, which is consistent with the intuition that the most probable number of heads becomes more and more likely as the number of tosses increases.

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