# Tagged Questions

Mathematical questions about Algorithms, including the analysis of algorithms, combinatorial algorithms, proofs of correctness, invariants, and semantic analyses. See also (computational-mathematics) and (computational-complexity).

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### A constrained topological sort?

Suppose that one has a directed, acyclic graph G, and each vertex $v$ contains a (positive) value $a_v$. Additionally, let $r$ be a constant. For my purposes, $r>1$, but this might not matter. ...
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### Algorithm to find primes up to $n$ in $O\left(\frac{n}{\log n}\right)$?

Consider the problem of given an integer $n$, generating a list of the primes not greater than $n$. An optimized version of the Sieve of Eratosthenes can do such task in $O(n)$, while the more modern ...
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### Product of primes mod n

Let $n$ be an odd composite number. I'm trying to compute $$f(n)=\prod_{n/2<p<n}p\pmod n$$ where $p$ ranges over the primes in the indicated region. Can this be done (significantly) faster ...
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### What's the most efficient algorithm for Divisibility?

What is the most efficient (in time complexity) algorithm known nowadays for the Divisibity Decision Problem: given two integers, say $a$ and $b$, does $a$ divide $b$? Let it be clear that what I ask ...
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### Solving general (dis)entanglement puzzles

What is the state of the art in (modelling and) solving a general (dis)entanglement puzzle? The following picture shows a nice example: There is a project called "The Untangler", which seems to be ...
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### Generating a Random Connected Graph

Given a graph G(V, E), with |V | = n and |E| = 0 (that is, the graph is empty), and a static set F containing all the possible edges. Consider the following algorithm for generating a random graph. ...
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### Is there a numeral system that makes both addition and multiplication easy?

Decimal positional notation, the system for writing numbers we all use every single day, makes addition very easy by transforming it from a computation to a repeated operation on individual digits (...
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### Traveling salesman problem: can a terrible strategy beat a good one?

Until yesterday, I was under the naive impression that constructing a weighted graph where the nearest-neighbour algorithm gives the worst possible route, would have the property that any other ...
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### Are algorithms for elimination of quantifiers over the reals practical?

I wanted to find the semialgebraic set in the $(a_0,a_1,a_2,a_3)$ space that guarantees that there exists at least one real root of the general polynomial equation of degree 4. For that purpose, ...
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### How to quantify the “uniformity” of a distribution of holes in a surface

I want to try to quantify if the distribution of holes over a surface is uniform or not. The holes can have any given shape and can be arranged in any way over the surface. Three examples are ...
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### Does Pollard rho works for Gaussian integers?

Should I expect that the Pollard rho method ...
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### Can I go from the LU factorization of a symmetric matrix to its Cholesky factorization, without starting over?

I mistakenly computed the LU factorization and then realized that the question is asking for a Cholesky factorization, i.e., finding a lower triangular matrix L such that the symmetric matrix A has ...
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### Analysis of sorting Algorithm with probably wrong comparator?

It is an interesting question from an Interview, I failed it. An array has $n$ different elements $[A_1 , A_2, ..., A_n]$ （random order）. We have a comparator $C$, but it has a probability $p$ to ...
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### Computing N cofactors

I have an $N\times N$ matrix of small size (say $N=20$). Is there a way to evaluate the $N$ cofactors of the elements of the first row, faster than in the obvious way as $N$ independent determinant ...
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### Algorithm for finding “fact families”

My friend's 3rd grader encountered the following question regarding "fact families" on her math homework: I was in 3rd grade sometime in the 1980s, so I don't believe I ever encountered this term ...
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### A summation involving the ceiling function

I'm trying to find a better method of calculating the sum $$\sum_{k=1}^N\lceil ak\rceil^2$$ where $a$ is an irrational number. So far, my only idea is to somehow use a best rational approximation. ...
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### Decoding of Gabidulin code

Consider the space of matrices in $\mathbb{F}_q^{n \times m}$ where $\mathbb{F}_q$ is the finite field with $q$ elements. We can define a metric on this space, given by $d(A,B) := rank(A-B)$, called ...
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### Is there a way to figure out the minimum number of participants or maximum number of rounds in my tournament style?

I just finished hosting a Euchre tournament at work that was meant to get people to meet other people in the company. This is the third time I've hosted this type of tournament. The first two times, ...
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### How to quickly approximate the eigenvectors of a symmetric matrix

Given a symmetric $n \times n$ matrix $A$, is there any algorithm that can quickly approximate all of its eigenvectors? By "quickly", I mean with time complexity less than $\mathcal{O}(n^3)$.
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### Nice puzzle: Creating a binary word using weights and scales

You are given $N$ weights where for each $i \in \{1,2,...,n\}$, the $i$-th weight weighs $i$ pounds. You are given an $N$-binary-word that's formed by $L$'s and $R$'s and scales. You need to provide ...
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### How to generate a function with multiple variables to fit experimental data?

I am researching methods to increase the accuracy of an algorithm that is currently used to analyse radiation patterns as they hit our sensor. (For the non-physicists, we will mainly see Alphas, ...
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### Implementing the Risch algorithm to integrate $\dfrac{\log(x)+2}{x^{2}\log^{3}(x)}$

Following the work of Andreas Wurfl i am trying to implement the Risch algorithm on $\int{\dfrac{\log(x)+2}{x^{2}\log^{3}(x)}dx}$ following his method for extensions that are purely logarithmic, we ...
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### Simple criteria to know if the p-nary notation of an integer can generate a tree by preorder traversing?

I am treating with a preorder tree traversal structure(which means sequences where the children of each tree node are listed behind it) now for some other problems and the structure is like: ...
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### Optimized way to compute L1 distance matrix

I'm computing distances between two groups of multi-dimensional points giving a matrix of distances pairwise between points. For the L2 (euclidean) distance I can use optimized matrix multiplication ...
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### Count swap permutations

Given an array A = [1, 2, 3, ..., n]: ...
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### Minimum number of vertex moves to un-intersect a polygon with itself

In my game, I have n points that form a self-intersecting polygon. The points can be moved by dragging them. How can I form a non-intersecting polygon this way, in ...
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### Binary Palindrome

Let an integer n be given. Write the integers from 1 to n in binary notation successively from left to right. In the resulting string consisting of zeros and ones, choose a palindrome substring of ...
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### Computational hard math problem

Given a square filled randomly with the numbers $1$ to $N$, for instance \begin{array}{cccc} 16 &12 & 9 & 1\\ 11 & 3 & 4 & 7\\ 2& 8 & 5&14\\ 6& 10& 15&...
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### Selecting k vectors with maximum spread out of a set of n vectors

Given a set $\mathcal{V}$ of $n$ vectors, find a subset $\mathcal{V}_k = \mathcal{V} - \mathcal{V}_{n-k}$ containing $k$ maximally spread vectors. Intuitively, these $k$ vectors should be spread as ...
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### Test for equivalence of algebraic expressions

We are looking for the most efficient (most recent, or best) techniques to check if two algebraic expressions (elementary, Calculus-type functions) are equivalent (or if an expression is equivalent to ...
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### What is the name of this algorithm for calculating the lowest common multiple?

I have implemented a quite slow algorithm to calculate the lowest common multiple of 20 different integer values. It is described here with an example : http://www.cut-the-knot.org/Curriculum/...
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### Algorithm For Continued Fraction of $\pi$ without error.

Is there an algorithm that will output the numbers in the continued fraction of $\pi$ without error? If one uses the usual method of calculating continued fractions, an approximation of $\pi$ is ...
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### Algorithm for generating homeomorphically irreducible trees of size n

In this video they talk about generating all the homeomorphically irreducible trees of size 10. I was wondering if there is a generating algorithm for generating all the homeomorphically irreducible ...
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### Use of graph theory to determine tensor contraction ordering

I am considering using a computer program to execute tensor contractions like the following: $\displaystyle \sum_{ij}^{o} \sum_{ab}^{v} \sum_{KL}^{X} B_{ia}^{K} B_{ia}^{L} B_{jb}^{K} B_{jb}^{L}$ ...
I am trying to understand quadratic sieve algorithm and now I am thinking of the way to check if number is smooth over a factor base? For example, say I have number $n = 87463$. First,I find bound \$B =...