Questions tagged [combinatorics]
Counting or enumerating elements in a set or other finite discrete structure.
345
questions
0
votes
1answer
27 views
How can I generate a completely balanced combination
Attaching a code that generates unique combinations of 8 letters but, it still lacks a condition wherein all letters should have equal counts per column. For my code, basically, letter a to h should ...
0
votes
0answers
10 views
How to create matrix of all 2^n binary sequences of length n using recursion in R? [migrated]
I know I can use expand.grid for this, but I am trying to learn actual programming. My goal is to take what I have below and use a recursion to get all 2^n binary sequences of length n.
I can do this ...
7
votes
1answer
428 views
Why does the likelihood function of a binomial distribution not include the combinatorics term? [duplicate]
So the likelihood function for a binomial distribution is:
Why is the likelihood function above not multiplied by a combinatorics term: n! / (x! * (n - x)!)
If the likelihood function is ...
0
votes
0answers
16 views
How many different looking selections can be made?
Problem: Consider 12 balls (3 orange, 3 green, 3 blue, 3 red). We randomly choose 9 balls from the 12. How many different looking selections can be made?
I only know that ${12 \choose {3,3,3,3} }$ is ...
0
votes
1answer
24 views
What's the probability that a license plate contains one A and one 7? [closed]
A license plate is composed of 4 letters and 3 digits. Whats the probability that a license plate contains at least one A and one 7?
2
votes
1answer
37 views
Why are probability problems involving combinations often solved indirectly?
Let's say you have a box with 25 cell phones in it, of which 2 are defective. If a person selects 10 cell phones at random, without replacement, what is the probability that both defective cell phones ...
1
vote
1answer
23 views
How to calculate the number of combinations that will add to a specific total
If I have for example 3 variables, and I need them to add to specific total, say 6, how do I calculate the number of combinations that will add to 6. Like 1+1+4=6 and 1+4+1=6 and 2+2+2=6 and 1+2+3=6 ...
1
vote
1answer
43 views
How many times I need to draw k items from a list of n items to get a probability P that each item is selected at least once? [duplicate]
I have seen a few related questions but not exactly what I am looking for (in particular this and this) I think. I may be missing something since I am only a beginner at these counting problems.
I am ...
3
votes
3answers
127 views
M buckets out of N have at least one ball
We have N buckets and we start filling them randomly with balls. At the end we know that we have exactly M buckets that have at least one ball in them. Both N and M are given and M < N.
What is ...
0
votes
1answer
39 views
Probability 4 places each get an ace when dealt a deck of cards - union approach
Problem: A deck of 52 cards is dealt to 4 players, each receiving 13 cards. What is the probability each player gets an ace.
I understand the combinatoric approach, but am unsure why the following ...
3
votes
1answer
70 views
When rolling a die $N$ times, what is the probability of an $X$ appearing before the last $Y$?
So a valid state is when there are both $X$ and $Y$ in the roll sequence, and the lowest index of an $X$ is smaller than the biggest index of a $Y$.
In particular, if after $N$ rolls, there is no $X$ ...
0
votes
0answers
20 views
Expected frequency of three-way combination given all two-way combination frequencies
Given an alphabet {A, B, C}, and observed frequencies of each element, a, b, and c respectively; I can calculate the expected frequency of the combination AB as a*b.
I want to know how to do the same ...
1
vote
1answer
34 views
Sampling inclusion probability for multiple items
Suppose we sample (uniformly, with replacement) $t$ times a set of $N$ items.
What is the probability $x$ that the sample contains $y$ different items?
This is representative of a real-life scenario ...
2
votes
1answer
21 views
Mixed order and unordered counting
If you care about ordered or unordered counts you can use nCr or nPr. But If you want a few in a certain order and a few unordered is there a general approach?
For example imagine you have 6 red ...
0
votes
0answers
12 views
Markov Chain Monte Carlo for Decryption Purposes
I've been reading the "Markov Chain Monte Carlov Revolution" paper by Diaconis and was intrigued by his application to decryption. I've done some further reading and am now trying to work through ...
1
vote
1answer
25 views
Number of Model Combinations
I have a model selection combinatorics problem.
My covariate pool consists of six covariates: A1, A2, A3, B, C and D. However, covariates A2 and/or A3 can only be included if A1 is also included. ...
0
votes
0answers
37 views
probability/counting problem
This is an even question from the textbook so unfortunately there's no solution. Suppose $\DeclareMathOperator{\P}{\mathbb{P}} S = \{1,2,3,4,5,6,7,8\}$ with $\P({s}) = 1/8$.
Part C) asks How many ...
0
votes
0answers
38 views
Formulating a mixed integer programming problem with a binary maximization term
Real-world description of the problem:
In the US, funding for school lunch programs is given to local school districts by the Federal government. If >=40% of students in a school or group of schools ...
0
votes
1answer
22 views
Probability of group wins in a lottery: Same versus different first digit in a lottery number drawn digit by digit
I want to calculate the probability to win in a certain lottery. These are the rules:
In the lottery, participants can participate individually or as a group. Groups can have up to 200 members.
Each ...
1
vote
1answer
38 views
Finding the expectation: Draws to get 2 gold coins
Omar has saved 11 precious coins, 7 of which made of gold, in a jar. He
draws the coins one by one from the jar, to find the gold coins. If $X$ is
the number of coins drawn until he has found the ...
0
votes
1answer
30 views
Drawing six cards from a deck of cards
Six cards are drawn from a well-shuffled deck. You are told that at
least an ace (A) is among the cards. What is the probability that
there is at least a king (K) or at least a queen (Q)? (A deck ...
2
votes
1answer
50 views
How to solve this combinatorial problem about selecting letters from different groups?
The English alphabet consists of 26 letters, 6 of which (A, E, I, O,
U, Y) are vowels and the remaining 20 are consonants. If each letter
has the same probability of being chosen, what is the ...
0
votes
1answer
140 views
Probability and Counting
Suppose You are ordering two pizzas.A pizza can be small,medium,large or extra large,with any combination of 8 possible toppings( getting no toppings is allowed, as is getting all 8) How many ...
1
vote
2answers
124 views
How to choose the best combination of covariates in Cox multiple regression?
I am performing a multivariate Cox regression analysis, and would like to find what combination of those covariates best predict my outcome.
Say I have a list of candidate genes whose expressions ...
2
votes
1answer
35 views
Item sampling techniques to reduce processing time for questionnaire
I have a well-crafted draft of a questionnaire, containing 240 items. The average processing time is 20 secs per item, so the entire questionnaire would take around 1 hour 20 min to complete. I want ...
0
votes
0answers
20 views
All different size groups to create m groups from n samples
Suppose we have $n$ samples that we want to divide in $m$ groups. How can we get the total number of groups and their sizes (if possible)?
Examples:
For $n = 6$ and $m = 3$ we would have $3$ groups as ...
1
vote
0answers
19 views
Counting the number of tuples $(i,i',k,k')$ satisfying some conditions
I'm struggling to find the cardinality of a set that will be presented below, as well as an upper bound for it. It is an engaging problem and perhaps not trivial.
I will give now the baseline of the ...
5
votes
2answers
122 views
What is the probability of having a pair of doubles when throwing dice?
What is the probability of having a double (two dice will show the same number)
when we throw 5 dice (all dice have 6 sides)?
My calculations are:
...
1
vote
0answers
9 views
Combinatorics of distinquishable sequences of events
I have found a problem that I can only solve by guessing, but without any systematic solution:
3 distinguishable balls (red, blue, green)
5 distinguishable sites (A, B, C & X, Y)
X, Y can ...
0
votes
0answers
14 views
Estimating weights in the assignment problem
How would you learn a function with the emphasis on feature interactions?
I have the standard assignment problem:
$$
\max_{x_{ij}} \sum_{(i, j)} w_{ij} x_{ij},
$$
where $w_{ij}$ is the weight of ...
0
votes
1answer
20 views
Find the probability that two samples of F unique items taken from a population of N unique items has at least one common item
What is the probability that two samples of F unique items taken from a population of N unique items has at least one common item?
0
votes
0answers
3 views
Calculating # of possible binary strings given constraint
Consider a random binary string where each bit can be set to 1 with probability $p$.
Let $Z[x,y]$ denote the number of arrangements of a binary string of length $x$ and the $x$-th bit is set to 1. ...
0
votes
1answer
21 views
Creating mean variables using all possible combinations of the original variables in r [closed]
I need to create a function that calculate the mean of all possible combinations of a given number of variables, and store the means values as separate variable and the number of variables whose means ...
1
vote
2answers
100 views
sample all pairs without repeats
Assuming I have a very large number of $K$ colored balls and we know the fraction of each color. If we randomly sample all pairs so that all pairs have two balls of different colors then what is the ...
0
votes
0answers
22 views
All combinations choosing 3 items each from 4 unique bags
Trying to do something like expand.grid in R, but as if expand.grid created all combinations of sets of 3 from each factor/group.
(E.G.)...
...
1
vote
3answers
139 views
How to set up an event with probability of one seventh from coin tossing?
I have been recently asked in an interview a question that: How to set up an event with probability of one seventh from coin tossing?
I couldn't find the correct answer and still don't know the ...
0
votes
2answers
51 views
combinatorics problem: two ways to solve same problem?
Currently i'm struggling alot with combinatorics.
The problem: There are four siblings. What is the probability that at least two of them are born in the same month?
This question asks to get two ...
0
votes
0answers
23 views
Probability of combination of letters appearing on multiple custom dice
Question:
Say I have a set of 24 dice that have different letters on them, what is the chance that in any roll a certain combination of letters appears at least once?
Combination I want:
{a, a, b, c, ...
2
votes
1answer
39 views
Modified coupon collector: killing traitors problem
You have $n$ people, $m$ of whom are traitors. You keep killing them randomly until all the traitors are gone (which you presumably have some way to verify). On average, how many people $X$ will you ...
0
votes
1answer
16 views
Why is the formula for permutations of size one n!
I am looking at this formula
I want to first make sure that I understand the principle. This is saying that, if I wanted to find out all the ways I could order the letter of the alphabet into a 26-...
0
votes
1answer
22 views
Does the multinomial coefficient create combinations or permutations?
I am looking at this formula
I just want to make sure I get it. For example, if I want to split a fifty-two card deck into all the possible combinations of four, thirteen card hands, then I get:
52!...
0
votes
1answer
705 views
Probability of normal random variable yielding highest value among other normal variables
Let's say I have a competition with $N$ participants. Each participant yields a score that is normality distributed with unique means and unique variations. Each participant gets to post one score (we ...
1
vote
0answers
31 views
Probability of having a match in an existing database [closed]
Imagine you are an airport authority in CountryLand and you have two passenger types: human passengers and animals.
There is no way for you to distinguish between human passengers and animals except ...
3
votes
0answers
58 views
Probability to be present in every subgroup
Assuming you randomly split a set of n elements in $y$ equally sized parts(each has $x$ elements). Now you have a subset of m elements in the set of n elements ($m> (y/2)$). What is the probability ...
0
votes
1answer
31 views
What is the probability that two samples from a source have no overlap?
I picked 29 results from a list of 429 results. I then picked a second group of 27 (with replacement) results from the same list of 429. There was no overlap between the two samples. What is the ...
0
votes
1answer
57 views
How to calculate number of solid color images on the screen of random pixel generator?
Is there a statistical formula/function to calculate number of basic geometric shapes on the displays of random pixel generator?
Let's say we have a basic random pixel generator which has 10*10 ...
1
vote
0answers
29 views
Birthday problem extension to unequal probabilities and multiple collisions
Let $p_1, ... ,p_k$ denote the probabilities of drawing bin $1, .. ,k$, where $\sum_{i = 1}^{k} p_i= 1$. My question is if we draw $n$ times, how can I show that the probability of no bin being drawn $...
0
votes
0answers
48 views
How many pairwise combinations is possible?
I have a dataset regarding a natural language problem that am tackling with machine learning. My goal here is the generate training samples for one of the classes. I do this by doing pairwise ...
0
votes
2answers
33 views
Checking my combinatorics
I am modelling a flash memory system where 0.3 requests are writes (and so take 100 cycles to complete) and 0.7 are read-only (and so take 50).
The system can handle 4 requests at once (the ...
2
votes
1answer
37 views
Probability of drawing different items from a subset with replacement
I'm trying to generalize a game mechanic, to understand the probability that the game developers are giving us, but this went over my head.
The mechanic is:
draw 10 items from a bag of 33 items
each ...
