# n*m distinguishable balls with m different colors, the probability of randomly choosing k balls containing all balls from at least 2 different colors

Assume we have m groups of n balls and the balls in the same group have the same color. So there are m*n balls in total. Now, suppose we randomly choose k>(2*n) balls from the set of m*n balls. How much is the probability that the chosen k balls contain all balls of at least two different colors (entirely all balls of two groups)?

In other words, the unchosen set of balls contains the balls of different m-2 colors at most (instead of m colors).

To grasp better, notice the picture of 3*4 ( = n*m) balls. Each group of 3 balls has the same color. The probability I am looking for is to choose k balls containing the balls of two entire groups. For example, choosing balls 1, 5, 9, 3, 7, 11, 8 (contains all yellow and blue balls).

I hope I could explain the problem clearly. I have implemented a simulator for testing different scenarios. Then I tested the simulated results with different combinatorial/binomial solutions. But I get different results every time and now I'm lost.

This is my simulator in python testing different choices many times:

from random import sample
from collections import Counter
m = 4
n = 3
k = 5
it = 100000
balls = range(m*n)
cf = 0
for i in range(it):
choices = sample(balls, k)
samecolors = map(lambda x:x%m, choices)
cnt = Counter(samecolors)
mc = cnt.most_common(2)
if (mc[-1][-1] == n): // if the second most common chosen color has *n* balls
cf += 1
print(float(cf)/float(it))

The overall number of ways to choose $$k$$ out of $$mn$$ balls is $$\binom{mn}{k}$$, where all balls are assumed to be distinguishable. Among them naively there are $$\binom mr\binom{mn-rn}{k-rn}$$ combinations consisting of at least $$r$$ full sets of balls of the same color. However if $$k\ge(r+1)n$$ the above expression will double-count all combinations consisting of more than $$r$$ full sets, which should be accounted for. The correct way for this is the generalised inclusion-exclusion principle: $$\nu_r=\sum_{j\ge r}(-1)^{j-r}\binom jr\binom mj\binom{mn-jn}{k-jn},$$ which gives the number of combinations with exactly $$r$$ full sets.

To obtain the number of combinations with at least $$r$$ full sets one should sum the above expressions: \begin{align} N_r=\sum_{i\ge r}\nu_i&=\sum_{i\ge r}\sum_{j\ge i}(-1)^{j-i}\binom ji\binom mj\binom{mn-jn}{k-jn}\\ &=\sum_{j\ge i}(-1)^j\binom mj\binom{mn-jn}{k-jn}\sum_{i\ge r}(-1)^{i}\binom ji\\ &=\sum_{j\ge r}(-1)^{j-r}\binom{j-1}{r-1}\binom mj\binom{mn-jn}{k-jn}. \end{align}

Thus, the probability in question reads (with $$r=2$$): $$p_r=\frac {\sum_{j\ge r}(-1)^{j-r}\binom{j-1}{r-1}\binom mj\binom{mn-jn}{k-jn}}{\binom{mn}k}.$$

• Thanks for your answer! It matches perfectly with the simulation results! May 15 '20 at 5:55
• @masoodgholami You're welcome!
– user
May 15 '20 at 7:08
• I posted a new question with regard to this one (a little bit more general) here: math.stackexchange.com/questions/3675906/… I would be happy if you could take a look at the question. thanks May 15 '20 at 9:18

Using multinomial theorem, the total number of cases are the number of solutions of the equation $$x_1 + x_2 + x_3 + ...+ x_m = k, x_i \in \{0, 1, ... n\}$$ The number of solutions is coefficient of $$x^k$$ in $$(1-x^{n+1})^m \times (1-x)^{-m}$$. This will have to be evaluated depending on the value of $$k$$. Let this number be $$A$$

The number of required cases are the number of solutions of the equation $$y_1 + y_2 + ... + y_{m-2} = k-2n$$ Note that we have 2 complete sets of $$n$$ elements selected, so we are finding the number of ways to select the remaining $$k-2n$$ elements. The number of solutions is coefficient of $$x^{k-2n}$$ in $$(1-x^{n+1})^{m-2} \times (1-x)^{-(m-2)}$$. Let this number be $$B$$.

The net probability is $$P = \frac{B}{A}$$

$$B$$ and $$A$$ would have to be calculated based on a numeric value of $$k$$.

• I will test this in my simulator, however, I should first find a function which calculates B and A. I will write the results thereafter. In the meantime, could you please explain your approach a bit more? how did you come up with the equations of A and B? May 14 '20 at 17:32
• "Note that we have 2 complete sets of n elements selected." Are you also counting the ways to select these 2 sets? That is, should the $B$ be multiplied by $\binom{m}{2}$ or am I missing something? May 14 '20 at 20:16
• Note that the number of solutions to $x_1+\dots+x_m=k,x_i\in\{0,1,\dots,n\}$ is given by: (See extended stars-and-bars problem(where the upper limit of the variable is bounded) answers.) $$\sum_{q=\lceil\frac{k+m}{n+1}\rceil}^m (-1)^{m-q}\binom {m}{q}\binom{q(n+1)-1-k}{m-1}$$ May 14 '20 at 20:27
• I wrote the eq(k,m,n) function that produces the result of your suggested equation (from the link), then by producing ( eq(k,m,n-1) + m*eq(k-n,m-1,n-1) ) / eq(k,m,n), I intended to get the probability of having no full colors at all in the choices + having one full color in the choices, then by dividing the result to the total number of choices I could have got the probability of zero or one full-colors in the choices, the complement of this number would have delivered me the needed probability in the problem. However, the results don't match the simulations. Am I doing something wrong? May 14 '20 at 21:22
• To answer you question, yes the counting ways would be important and thus the result should be multiplied by (m 2). May 14 '20 at 21:22