DNS Spoofing/Poisoning
An Introduction |
Prof. David Bernstein
|
Computer Science Department |
bernstdh@jmu.edu |
Assuming all birthdays are equally likely, the probability that an individual has the same birthday as you is \(\frac{1}{365}\). Hence, the probability of the complement (i.e., the individual doesn't have the same birthday as you) is \(1 - \frac{1}{365} = \frac{364}{365}\).
So, assuming independence, the probability that \(n\) people don't have the same birthday as you is \((\frac{364}{365})^n\).
Hence, the probability of the complement is \(1 - (\frac{364}{365})^n\).
So, the probability that someone has the same birthday as you for \(n=10\) is about 0.027, for \(n=30\) is about 0.079 and for \(n=50\) is about 0.128.
Let \(B_i\) denote the birthday of individual \(i\) and suppose we know the birthday of individual \(1\). Then, as we just saw, the probability that individual \(2\) has the same birthday as individual \(1\) is \(P\{B_2 = B_1\} = \frac{1}{365}\). Hence, the probability of the complement is given by \(P\{B_2 \neq B_1\} = 1 - \frac{1}{365} = \frac{364}{365}\).
The probability that 3's birthday is the same as 1's or 2's, given that 1 and 2 do not have the same birthday, is given by:
\[P\{B_3 = B_2 \text{ or } B_3 = B_1 | B_2 \neq B_1\} = \frac{1}{365} + \frac{1}{365} = \frac{2}{365}\]
Hence, the probability of the complement is given by:
\[P\{B_3 \neq B_2 \neq B_1 | B_2 \neq B_1\} = 1 - \frac{2}{365} = \frac{363}{365}\]
and:
\[P\{B_3 \neq B_2 \neq B_1\} = P\{B_3 \neq B_2 \neq B_1 | B_2 \neq B_1\} \cdot P\{B_2 \neq B_1\} = \frac{364}{365} \cdot \frac{363}{365}\]
Generalizing, the probability that \(n\) individuals have different birthdays is given by:
\[\frac{364 \cdot 363 \cdot (365-n+1)}{365^{n-1}}\]
Multiplying by \(\frac{365}{365}=1\), this probability is given by:
\[\frac{365 \cdot 364 \cdot 363 \cdot (365-n+1)}{365^{n}}\]
This can be re-written as:
\[\frac{365!}{(365-n)!} \cdot \frac{1}{365^n}\]
Thus, the probability of the complement (i.e., that any two people share a birthday) is just:
\[1 - \left(\frac{365!}{(365-n)!} \cdot \frac{1}{365^n}\right)\]
So, the probability that any two people share a birthday for \(n=10\) is about 0.117, for \(n=30\) is about 0.706 and for \(n=50\) is about 0.970.
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