Questions tagged [combinatorics]
Questions related to combinatorics and discrete mathematical structures
495
questions
3
votes
1answer
57 views
Problem in downvote system
Problem
For my game, I'm building a system where players have power/weight, and they can downvote each other, players with 66% of downvote weight are banned. The weight of the votes is calculated ...
-2
votes
0answers
18 views
computation of a hypergeometric sum [closed]
Let $q,\ell, M \in \mathbb{N}$
I am not familiar with mathematica, maple or mathlab to, at least see what this sum suppose to be. I am wondering if someone can compute this sum
$$
\sum_{m=0}^q\binom{q-...
0
votes
0answers
27 views
Algorithm and Time Complexity for k-Sum problems
In fact, there are three different k-Sum problems:
Problem1: Given unsorted integer array $\{a_1, a_2, ..., a_n\}$ and a target number $T$, determine whether there exist at least one solution $\{a_{...
0
votes
0answers
6 views
Count compound words with an ambiguous decomposition
I have a set of words $D$, and I make compound words by concatenating a fixed number $n$ of words from $D$ (repetitions are allowed). Let's call such words $n$-compounds. I want to know how many ...
3
votes
2answers
52 views
Which order is “lexicographic order”?
To choose 3 items out of 5 items [1, 2, 3, 4, 5], Donald Knuth's lexicographic algorithm (The Art of Computer Programming, Vol 4A, 2011, p. 358, Algorithm L (...
2
votes
0answers
30 views
8-puzzle problem [duplicate]
8-puzzle problem: The puzzle consists of an area divided into a grid, 3 by 3. On each grid square is a tile, except for one square which remains empty. A tile that is next to the empty grid square can ...
1
vote
1answer
27 views
Recover boolean vector from dot products
Question:
I want to determine a boolean vector $b \in \{0,1\}^n$ consisting of zeros and ones, but cannot access it directly. I can only call a black-box computer code which will take the dot product ...
1
vote
1answer
23 views
Selecting the right partition in NAUTY
Graph isomorphism solver Nauty has two main procedures, individualization and refinement, to get to a discrete partition. During refinement procedure, we take some cell of the current partition and ...
0
votes
0answers
18 views
maximum eigenvalue across subsamples
I have an $N$-dimensional vector of data, say $X_{t}$, with $1 \leq t \leq T$.
Of this vector $X_{t}$, I want to consider sub-vectors, say $X_{t}^{b}$, which are $m$-dimensional combinations of ...
0
votes
0answers
28 views
count all possible paths of length n in an undirected graph with use of dynamic programming [duplicate]
Given is an infinitely large grid graph. Use dynamic programming to calculate the number of possible paths
of a given length n from a given start node, so that fjor every path applies:
a) no vertex ...
4
votes
2answers
65 views
Count paths of length $n$ that a player can take
I'm writing a video game, and I'm trying to find an efficient way of calculating this. The goal is to count the number of paths of length $n$ that a character can take, where the character can move ...
2
votes
1answer
57 views
Conditions for a binary tree being balanced
Prove or disprove for each of the following two properties, whether a family of trees that satisfy the property is balanced.
If you disprove, the counterexample should consist of an infinite ...
1
vote
1answer
116 views
combinatoric grouping optimization problem based on time interval overlap, weight constraint, and distance minimization
Let each element be an individual. Consider that an individual is defined such that each individual has a time range, weight, and location.
The goal is to group together individuals whose time ranges ...
5
votes
2answers
137 views
Finding row wise sum of transpose of hv-convex binary matrix
I'm stuck on a problem involving the Gale-Ryser Theorem. The problem's input gives me the row-wise sum of an hv-convex binary matrix(n*m).
...
2
votes
2answers
46 views
Number of possible heaps on $\{1,…,2^h-1\}$
Let $C_h$ be the number of possible heaps for the set of keys $\{1,...,2^h-1\}$. Determine a recurrence relation for $C_h$ via the substitution method and prove it.
Definition
A binary tree ...
1
vote
0answers
63 views
Tree Optimization, Combinatorics, algorithm [closed]
My Partners and me, we are trying to optimize frequency process...
I used Java to show our Problem, but the question is about algorithm NOT about Java implementation. Although implementations in java ...
2
votes
0answers
24 views
Linear order minimizing weighted distance from special element
Let's say I have a set of beads, $b_0,\dots,b_n$, and let $b_0$ be the 'special bead'. I want to lay out the beads on a string to minimize the total cost, defined as $\sum_{i=1}^n w_i \cdot d(b_0, b_i)...
1
vote
0answers
66 views
Approporiate algorithm for a graph theory problem
So I have recently ran into a graph theory problem and was unable to find a matching algorithm for the problem or reword the problem to match some existing algorithm.
The problem is pretty ...
1
vote
2answers
49 views
Maximal cliques in a multipartite graph - efficient?
I am looking at a combinatorial optimisation problem where I have N classes and k objects of each class.
Now I am looking for the optimal subset such that each of the N classes is represented ...
0
votes
0answers
31 views
Minimum Ratio Spanning Tree
Problem statement:
Given an undirected graph $G = (V, E)$ with edge $e_i$ having two associated positive values $c_1, c_2$. Find a spanning tree $ST$ such that (ratio of the spanning tree): $$\frac{...
5
votes
0answers
48 views
Number of strings at given edit distance
I would like to know the number of strings at edit distance $n$ of a string $s$.
I guess this is textbook knowledge... but I cannot find the textbook in question.
More formally, I have an alphabet $\...
0
votes
1answer
54 views
How many possible ways to go right and up in an array
Let's say we have a 2D matrix, and we begin at $(0, 0)$.
We must travel $m$ steps to the right and $n$ steps up, in any order. Each step moves the position right or up by $1$.
For example if $n = 5$...
0
votes
1answer
44 views
Finding combinations of variables that can take value of -1/0/1 that produce sum of 0 with added constraint
I have 64 variables that can either take a value of -1, 0, or 1 and I am interested in finding all possible combinations of variables such that I have n variables in each the positive and negative ...
6
votes
0answers
107 views
Algorithms to generate random nowhere-neat rectangulation?
I want to generate random rectangular partition of a given $m*n$ rectangle under the constraint that it must be nowhere-neat partition. Nowhere-neat partition means that a dissection of a rectangle ...
1
vote
1answer
93 views
How to calculate combination from given n,r and rank?
Suppose that $S=\{1,2,...,n\}$ and we are given an integer $r\leq n$.
An $r$-combination of $S$ is obtained by selecting $r$ distinct integers out of the $n$. We order all $r$-combinations for a ...
1
vote
1answer
12 views
Software metric for data growth
I'm writing a paper for some software that uses combinatorics to generate large result sets. I would like to describe that if I put in $n$ elements, I will get in return $2^n$ elements.
Is there a ...
0
votes
0answers
27 views
Handling $AND$ and $OR$ cases in MILP?
Suppose I want to have an integer program for handling the cases
$x_1>1\wedge x_2>1\wedge x_3>1\wedge\dots\wedge x_n>1\iff\delta=1$
$x_1>1\vee x_2>1\vee x_3>1\vee\dots\vee x_n&...
2
votes
1answer
59 views
Maximal size of a set of ordered words such that no pair of letters occurs twice
Consider an alphabet $\Sigma=\{1,\dots,n\}$.
An ordered word is a word $w=w_1w_2\dots w_k\in\Sigma^*$ such that $w_1<w_2<\dots<w_k$. In other words, an ordered word is a strictly increasing ...
0
votes
1answer
41 views
When do we use parallel algorithms for enumerating combinations?
I know that combination is used in many areas. But do we really need parallel version of algorithms for that? If so, where do they used?
Here is a famous example of parallel algorithms, Adaptive and ...
2
votes
1answer
123 views
Permutation of n-size array with possible repeated elements. E.g [1, 2, 1]
What would it be a recursive algorithm to get permutations for any list of n elements that might contain or not repeated elements?
For the following 3-element list ...
0
votes
1answer
39 views
invariant of bin packing
We are given an array of integers and a number K. We need to pack these integers into bins. The condition is that we have to use exactly K number of bins and each bin should have equal capacity. We ...
1
vote
1answer
65 views
Counting models satisfying a boolean formula
I'm trying to implement the #2-SAT algorithm from the paper "Counting Satisfying Assignments in 2-SAT and 3-SAT" (Dahllöf, Jonsson and Wahlström, Theor. Comput. Sci. 332(1–3):265–...
9
votes
2answers
171 views
Find an optimal ordering
I came across this problem and am struggling to find a way to approach it. Any thoughts would be greatly appreciated!
Suppose we are given a matrix $\{-1, 0, 1\}^{n\ \times\ k} $, for example,
...
0
votes
1answer
42 views
Number of possible min heaps
The number of possible min-heaps containing each value from {1, 2, 3,
4, 5, 6, 7} exactly once is --------------
According to me, the answer should be 48. The first element 1 is fixed as root. The ...
1
vote
1answer
23 views
Minimum number of tree operations to normalize a labeled tree
Given a binary tree with labels on the leaves, like $(bc)(ad)$ or $((af)e)(c(db))$, which we can interpret as a product of terms with respect to a commutative associative operation, how many ...
3
votes
0answers
73 views
Adjacent Gray code
Gray code is permutation of $\{0,1,2,\dots,2^n-1\}$ such that each of consecutive number is differs only one bit in binary representation.
Example for $n = 3$
$000\\
001\\
011\\
010\\
110\\
111\\
...
3
votes
1answer
46 views
Sampling numbers from a weighted set that sum to constant value
So I have a multi-set of positive integers $S = \{n_1, n_2, \dots\}$ with associated weights $W = \{w_1, w_2, \dots\}$. I want to sample some numbers, without replacement, from $S$ according to ...
2
votes
2answers
88 views
Efficient n-choose-k random sampling
Is there an efficient method of sampling an n-choose-k combination at random (with uniform probability, for example)?
I have read this question but it asks for generations of all combinations, not ...
1
vote
1answer
69 views
Placing items into compatible bucket types to find an optimal total value
Suppose we have a list of buckets, each with a unique type and a maximum capaciy. We also have a list of items, each with a value and a list of compatible bucket types. An item is compatible with a ...
2
votes
1answer
34 views
Generate a random combination in O(k) time and space?
How to generate a random combination of $k$ numbers from $n$ choices in $O(k)$ time and space, if we can generate a random number between 1 and $O(n)$ in $O(1)$ time?
I know only 3 algorithms: with $...
0
votes
0answers
19 views
Combinatorial Optimisation of a College Timetable
I'm a relative newcomer to the world of combinatorics, and would like a suggestion for how to tackle this problem.
We have an event that 750 students will be attending. The event is split into 4 ...
0
votes
1answer
181 views
What is the fastest algorithm of generating all possible permutations (within a given set of constraints) of a multidimensional array?
There is D-dimensional array A. The number D and the size Sd of every dimension d=1..D is input from keyboard. There is also 1-dimensional array E of size N. It consists of unique integer numbers 0..N-...
2
votes
1answer
51 views
Subset on boolean cube with largest sum of biases
On the boolean cube $\mathcal{B}=\{0,1\}^n$, we assign each vertex a value by $p:\mathcal{B}\rightarrow[0,1]$. Let
$$\tilde{p}_i=\sum_{x\in\mathcal{B}}(-1)^{x_i}p(x).$$
What is the value of $\max_p\...
8
votes
1answer
70 views
Given a constant k, find the biggest possible rooted tree, if for every path from root to leaf, the sum of the arity of its nodes equals k?
As an example, here are all possible trees for the case $k=3$:
On each node written is its arity (= the number of children).
While this should be solvable by dynamic programming, I think there was a ...
1
vote
1answer
18 views
Bounds on “well dispersed” sparse matrices
Suppose we have an $n\times n$ zero/one matrix $M$, with $k$ ones. Let us say that the extent of $M$ is the maximum of $i+j$ over all ones at positions $(i,j)$ of the matrix, and the quality $q(M)$ is ...
2
votes
1answer
45 views
Why 2 different edge min-cuts in an undirected multigraph must be completely disjoint?
For the proof of a maximum of (n 2) min-cuts in any n-vertex undirected multigraph using the random contraction algorithm, we need to know that no min-cut shares an edge with another different one.
...
1
vote
0answers
43 views
Find a partition of multiset of binomial coefficients with constriants
Given the multiset $S$ where the elements are defined by the binomial coefficient ${n \choose k}$ where $n \in \mathbb{N}$ and $
0\leq k \leq n$ find the partition $P$ of $S$ such that the sum of ...
0
votes
0answers
43 views
O(1) algorithm to get approximate number that is larger as a binomial coefficient
Ideally I need a to calculate the binomial coefficient ${p \choose n}$. But since the fastest algorithm to do this is an $\mathcal{O}(n)$ algorithm I would look to do something different. I don't ...
0
votes
1answer
33 views
Generation of all combinations of a set in max-differing/random order
I'm looking for an algorithm that generates all k-combinations of a set, such that each successive combination generated differs as much as possible (or in practice, a lot) from all previous ...
0
votes
0answers
59 views
Counting chords intersections in a circle
The problem is:
Given 2n distinct endpoints of n chords on the unit circle, count the number of intersections between chords (if k chords intersect at one point,
that point counts as $\binom{n}{2}$ ...
