To the untutored mind it seems like magic, or the greatest, weirdest coincidence imaginable. But it isn't magic, it's mathematics according to Richard Mak, and he's a bit like Near when it comes to the old calculations.
Death Note Light Yagami Actors' Shared Birthday Phenomenon
To see where and how each played Light, consult:
Casting Kira: Light Yagami Actors Across the Globe
Krisztián Kolovratnik and Tatsuya Fujiwara share a birthday on May 15th
Kenji Urai and Masataka Kubota share a birthday on August 6th
Yeong-Seon Kim and Nat Wolff share a birthday on December 17th
For the sake of simplicity let's assume that February 29th doesn't exist, which shouldn't affect calculations too much, but also that somebody is equally likely to be born on every day of the year. The latter is obviously not true because there are certain times of year where more babies are born than others for various reasons, but it still shouldn't take away the basic argument here.
Let's first think about the probability that *nobody* shares a birthday - after all, if you know the probability that nobody shares a birthday, you know the probability that at least two people share a birthday! Consider this:
- Person 1 doesn't have anybody to share a birthday with yet.
- Person 2 has a 364/365 chance to not be born on the same day as person 1.
- Person 3 has a 363/365 chance to not be born on the same day as person 1 and 2.
Another way of looking at it which may be easier to understand is this:
Suppose you have a room filled with N people [Matti helpfully supplies image to illustrate point - see right], where N is any whole number (because you can't have half a person, unless it is a corpse...).
That means that in a room of 23 people, or N=23, then person 1 (23-1) has 22 people to check his birthday with, person 2 (23-2) has 21 people to check his birthday with, and so on... You end up having 253 possible combinations of people, and so 253 possible birthday checks, which is a lot!
By these maths, it's actually pretty unexpected that the twenty-two aforementioned Light Yagami actors have three pairs of people sharing a birthday.
Actually working out specific numbers is quite difficult. I took a quick look at my university's digital library and couldn't find any research papers on anything beyond the canonical birthday problem (i.e. purely looking at the probability of two people sharing a birthday).
Nevertheless, the Kira Actors Birthday Phenomenon pings against a known mathematical problem, and the probability pans out.