- #1
AntiElephant
- 25
- 0
Homework Statement
I have [itex]N[/itex] balls and [itex]B[/itex] baskets. I look at each basket [itex]i[/itex] in turn and decide with some probability [itex]P_i[/itex] whether to insert one ball. After [itex]N[/itex] balls are inserted, I stop. If I look through all the baskets [itex] B [/itex], I stop; I do not make a second run-through, even if all the balls haven't been inserted. Can I write [itex]P_i[/itex] in such a way that, overall, each basket has an equal probability (which I'll call [itex]E_i[/itex]) of obtaining a ball?
Homework Equations
The Attempt at a Solution
This isn't a homework question so I don't know how difficult this could get. I'm finding it hard to generalise [itex]P_i[/itex] for all [itex]N[/itex] and [itex]B[/itex]. If [itex] N \geq B[/itex], then [itex]P_i[/itex] can just be a constant number and all baskets have the same probability, because there are enough balls to supply for each basket and the chance isn't affected by whether balls have been inserted in previous baskets. So the assumption is that [itex] N < B [/itex]
[itex]P_i[/itex] seems like it depends on three variables:
[itex] N [/itex]: The number of balls at the start.
[itex] B [/itex]: The number of baskets.
[itex] R [/itex]: The number of balls remaining when we reach [itex]i[/itex].
Obviously all [itex] P_i = 0[/itex] when [itex] R = 0 [/itex]. The problem would be easier if I could reason that maybe [itex]P_i[/itex] doesn't depend on [itex]R[/itex] when [itex]R>0[/itex], but I haven't been able to convince myself whether that's true.
The case where [itex]N = 1[/itex] is simple since I don't have to worry about the variable [itex] R [/itex]:
[itex]E_1 = P_1[/itex]
[itex]E_2 = (1-P_1)P_2[/itex]
...
[itex]E_i = (1-P_1)(1-P_2)...(1-P_{i-1})P_i[/itex]
...
[itex]E_B = (1-P_1)(1-P_2)...(1-P_{B-1})P_B[/itex]
One can use this to notice:
[itex] (1):[/itex] [itex](1-P_i) = P_i/P_{i+1} [/itex]
Our assumption:
[itex]E_1 = E_2 = ... = E_i = ... = E_B = 1/B[/itex]
Therefore
[itex] P_1 = 1/B [/itex]
[itex] P_2 = 1/(B-1)[/itex]
[itex] P_3 = 1/(B-2)[/itex]
Then use induction:
Assume
[itex] P_i = 1/(B-(i-1)) [/itex]
Show
[itex] P_{i+1} = 1/(B-i) [/itex]
Which is easily shown with point [itex] (1) [/itex] above. Leading me to the answer:
[itex] P_i=(1/(B−(i−1))) [/itex]
The case for generalising this to any [itex] N [/itex] seems very hard though, especially since the formula for each [itex] E_i [/itex] isn't just one large multiplication like it was with [itex]N=1[/itex], and is instead an addition of probability of all the permutations in which [itex] i [/itex] obtains a ball, and those permutations don't even have the same probability of occurring. For instance with [itex]N =2[/itex] for [itex]E_3[/itex] and [itex]P_i(R)[/itex] I'm looking at:
[itex]E_3 = (1-P_1(2))(1-P_2(2))P_3(2) + P_1(2)(1-P_2(1))P_3(1) + (1-P_1(2))P_2(2)P_3(1) [/itex]
It just becomes a mess to generalise. Is this possible? Can anyone provide some pointers for approaching this? Is there a name for this type of problem?
Last edited: