# Tagged Questions

For questions about the study of finite or countable discrete structures, specifically how to count or enumerate elements in a set (perhaps of all possibilities) or any subset. It includes questions on permutations, combinations, bijective proofs, and generating functions.

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### In AB + BC + AC = N, how can I find all possibilities for A, B and C in less than n³ computational time?

The problem is the one on the title. Given a N, find all possibilies for A, B and C that make this true: $AB+BC+AC = N$when $A \ge B \ge C$. This code in C do the job: ...
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### In how many ways can a group of $n$ people composed of six types be created with restrictions?

Suppose we need to create a group of $n \geq 20$ people with the following types and requirements: Scientists, at least 2; Pro Athletes, at least 1; Mathematicians, at least 5; Plumbers, at least 0; ...
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### Find the number of sums that you will get A, when A=100?

Problem: Take any number $A \in \Bbb{N} = \{1, 2, 3, \dots\}$, and then take $x, y \in \Bbb{N}$, where $x \ne y$ and $x + y = A$. Find the number of possible choices for $x$ and $y$ when $A=100$. ...
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### b) How many onto functions are there from A to C? [duplicate]

Let $A=\{1,2,3,4\}$ Let $B= \{a,b\}$ Let $C= \{ \text{hiking, baseball, hockey} \}$ a) How many onto functions from A to B $(1,a) (1,b) (2,a) (2,b) (3,a) (3,b) (4,a) (4,b)$ Thus 8. b) How many ...
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### Find least number of radial-subgraph of a graph

Background: Here is a group G of a people, one maybe another's friend. How to select least number of people to be a leader of a subgroup, so that everyone in the group G has a friend as a leader? ...
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### Combinatorial proof of $\sum_{k=0}^{n}(-1)^{k}\binom{n}{k}(l-k)^n=n!$, using inclusion-exclusion

If $l$ and $n$ are any positive integers, is there a proof of the identity $$\sum_{k=0}^{n}(-1)^{k}\binom{n}{k}(l-k)^n=n!\;$$ which uses the Inclusion-Exclusion Principle? (If necessary, ...
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### Consider the system S which can take n input parameters and each parameter can take on m values

(a) What is the maximum number of pairs a single test case for this system can cover? "I know that there are m^n different combinations in this example, but i'm unsure how many pairs a single test ...
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### Combination Problem : $6$ Countries , $4$ players from each country

$6$ Countries participate a world tournament . Each country has $4$ players. One Cricket player , One Rugby player , one Volleyball player and one Football player. Need to select a team of $8$ ...
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### Least amount of repetitions s.t. probability greater than 1/2

Assume that for a formula $F$ over $n$ variables, there are exactly $k$ allocations that satisfy it. How many random samples from the set $\{0,1\}^n$ are necessary to find an allocation satisfying the ...
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### N is a four digit number. If the leftmost digit is removed, the resulting three digit number is 1/9th of N. How many such N are possible? [closed]

N is a four digit number. If the leftmost digit is removed, the resulting three digit number is 1/9th of N. How many such N are possible with solution?
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### Probabilities in infinite Bernoulli type of series

While I was trying to solve the 1st problem from here I run into the following problem: find the probability of the events such as $1122213$ or $2122111116$ in infinite series of dice rolls which end ...
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### Problem 48 in A First Course in Probability

I have an issue with problem 48 Chapter 2, page 51 in Sheldon Ross' A First Course in Probability (9th edition). The problem is as follows, Given 20 people, what is the probability that among the 12 ...
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### Prove that $\dfrac{b^{n-1}a(a+b)(a+2b)\cdots(a+(n-1)b)}{n!}$ is an integer

Let $a$ and $b$ be integers and $n$ a positive integer. Prove that $$\dfrac{b^{n-1}a(a+b)(a+2b)\cdots(a+(n-1)b)}{n!}$$ is an integer. Define $v_p(x)$ such that if $v_p(x) = n$, then $p^n \mid x$ but ...
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### Transposal generators like {1, 1, 2, 3, 3, 2}

The sequence {1, 1, 2, 3, 3, 2} generates all the transposals of {1,2,3}. Just cyclically pick positions $n, n+2, n+4$. Is there a sequence like this for 1-4, 1-5, and so on?