ext_8145 () wrote in 2013-11-19 12:13 pm (UTC)

#### no subject

There was a similar question on facebook a while ago:

I wonder how many facebook friends you need so that you are more like than not to have a friend with a birthday on every single day of the year?

This was solved neatly by Ben:

Under (presumably) the same assumptions as above (uniform distribution of birthdays, ignoring leap years):

Define P(N,D) to be the probability that N people have exactly D different birthdays. Then we want to find the smallest N* such that P(N*,365)>0.5.

I'm thinking induction is the way to go. Suppose we have P(N-1,d) for all d, i.e. we know the chance of N-1 people having exactly d birthdays (for d = 1, 2, ... N-1).

Then to get P(N,D) there are two ways:
1) The N-1 people had D-1 different birthdays and the Nth person has a different birthday. The chance of this is (365-(D-1))/365 = (366-D)/365.
2) The N-1 people had D different birthdays and the Nth person has the same birthday as one of them. The chance of this is D/365.

So the recursion is
P(N,D) = [ P(N-1,D) * D + P(N-1,D-1)*(366-D) ]/365
with initial conditions P(1,1) = 1 and P(1,D) = 0 for D>1.

You can probably solve this with generating functions, but to be honest I don't care that much! A simple bit of code gives N* = 2287. This is pretty close to Vincent's answer. I wonder what happens if Vincent computes the median, and not the mean? Of course, since I said the code was simple, it's also quite possible that I made a mistake!

***

I think this is probably the solution for this... define P (N,X) as the probability after buying N Octonauts you have X Octonauts that you want? (I did this as P(N,O) with O for Octonaut initially, but it's _awful_ notation, because O looks like 0) Then we could have the recursion as P (N,X) = P(N-1,X) * P(didn't get an octonaut you want, given you already have X) + P (N-1,X-1)* P (did get an octonaut you want, given you already have X-1)

P(didn't get an octonaut you want, given you already have X) is easy, it's (X+3)/8 (so as soon as you have all 5, you never get an octonaut you want). Likewise P (did get an octonaut you want, given you already have X-1) is (6-X)/8

But I don't know how to actually do the recursion and calculate the answer...

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