Questions tagged [matrices]
For questions about construction and modification of matrices, objects represented by 2-dimensional arrays that are used to define linear operators within linear algebra.
289
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22
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Modify this GEPP algorithm to work with matrices
I've been stuck with this problem for a day or so, and I really can't figure it out. I need to modify this pseudocode so that it works when b is not just a vector, but a matrix with same number of ...
0
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0
answers
32
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Find submatrix with sum as close to k as possible
What is the efficient algorithm to find a submatrix (must be rectangle) with a sum that is as close as possible to k? Matrix consists only of nonnegative integers. Iterating through all possible ...
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0
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31
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Maximize sum of matrix after deleting K rows and K columns
You're given a m by n matrix filled with positive integers, as well as some integer k (0 <...
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1
vote
1
answer
143
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Time Complexity of Matrix Fibonacci Algorithm
In my reference, Exercise 0.4(e), Algorithms by Sanjoy Dasgupta, Christos H. Papadimitriou, and Umesh V. Vazirani, it is given that
$$
\begin{bmatrix}F_n\\F_{n+1}\end{bmatrix}=\begin{bmatrix}0&1\\...
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3
votes
0
answers
67
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What's the fastest known non-galactic algorithm for matrix multiplication of large matrices
"A galactic algorithm is one that outperforms any other algorithm for problems that are sufficiently large, but where "sufficiently large" is so big that the algorithm is never used in ...
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2
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1
answer
72
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A version of Bareiss algorithm or similar for symmetric matrices
A linear equation $Ax=b$ can be solved by reducing the matrix $A$ to upper triangular form by using Gaussian elimination or LU decomposition. If $A$ is symmetric and positive definite one can use ...
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1
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0
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36
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Algorithm for the inversion of a striped matrix with tridiagonal stripes
I'd like to compute the inverse of a matrix of size $S^2N \times S^2N$ over complex numbers that is composed of tri-diagonal $S\times S$ size blocks of tri-diagonal $S\times S$ block matrices. This ...
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35
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Matrix Multiplication with a matrix consisting of a single shifted row
I have the following question (prefacing this with the fact that this is a question from an exam, I am currently studying but am stumped so reaching out for help).
The answer to the following ...
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2
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1
answer
67
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Selecting a submatrix of a binary matrix NP hard?
I have the following problem and I am wondering if it is NP Hard or not.
Let $A$ be a binary matrix whose rows and columns are indexed by the sets $\mathcal{I}=1,...,m$ and $\mathcal{J}=1,...,n$.
A ...
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0
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0
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382
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How to generalize MATRIX-MULTIPLY-RECURSIVE to multiply n × n matrices?
the question is as follows:
"Generalize MATRIX-MULTIPLY-RECURSIVE to multiply n × n matrices for which n is not necessarily an exact power of 2. Give a recurrence describing its running time. ...
1
vote
1
answer
155
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Reorder columns in a 2d matrix to maximize the count of all repeated subarrays across all rows
I have a matrix (input):
--
c1
c2
c3
r1
AA
BB
CC
r2
CC
RR
BB
r3
EE
DD
FF
r4
KK
DD
EE
r5
DD
GG
KK
r6
PP
QQ
KK
Let's call each matrix cell a namespace. If two ...
1
vote
1
answer
30
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Writing an Algorithm to Represent a Bit Matrix in Minimal Operations?
I am trying to come up with an algorithm to find the minimal representation of the transformation from a zero matrix to a target matrix.
Specifically, I have an empty matrix and can perform operations ...
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1
vote
1
answer
91
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Given a binary matrix, find the number of sub-matrices with all ones
Given a matrix A, let Aij denote the element of the i'th row and j'th column.
$$ A_{i,j}\in \{0, 1\}$$
Find the number of sub-matrices with all ones.
1 <= #rows, #columns <= 150
P.S. This ...
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2
votes
0
answers
62
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Permuting matrix entries to lower rank
Suppose I have a rank-$k$ matrix $A \in \mathbf{R}^{m \times n}$. Now suppose this matrix has its elements shuffled by an adversary to maximize the rank. Is there a way to reverse this permutation and ...
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1
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0
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41
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Adding two 2D matrices together: row by row vs column by column
When adding two 2D matrices of the same size (in row major format), in sequential code with no vector operations, is it faster to add them column by column or row by row?
At first I thought it would ...
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8
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5
answers
3k
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In algorithms for matrix multiplication (eg Strassen), why do we say n is equal to the number of rows and not the number of elements in both matrices?
In Strassen's algorithm, we calculate the time complexity based on n being the number of rows of the square matrix. Why don't we take n to be the total number of entries in the matrices (so if we were ...
2
votes
2
answers
192
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Complexity of unbounded Gaussian convolution
What is the asymptotic complexity of doing a convolution of an unbounded Gaussian on an NxN input matrix $M$?
The naive approach is $O(n^4)$ ($R_{ij} = \sum_{k=1}^{n}\sum_{l=1}^{n}M_{kl}*g(\sqrt{(k-i)^...
- 1,404
1
vote
2
answers
96
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Convert lower-left matrix triangle 1D index to row, column
How can I convert a 1D index in the lower-left triangle of a grid into a row and column?
For example, consider this table of 1D indices, indexed by row and column
...
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1
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1
answer
38
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Expected value of maximum of a matrix of size $n$
I have a square matrix (call it $A$) of size $n$ ($n$ is a positive integer). Each column is a permutation of $[1:n]$.
I take the first row of $A$, i.e. $A(1,:)$ and wonder what will be the frequency ...
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0
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28
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Fast compute of F*P*FT matrix product
Let $P$ be a symmetric (positive definite, if that helps...) matrix of size $n$.
Let $F$ be a matrix of size $n$.
Is there an existing efficient algorithm implementation to calculate $FPF^T$ ?
Is ...
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0
votes
1
answer
30
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Split matrix by groups of columns but *capture all combinations of X columns* for some X
Say I have a big matrix, ~50000 rows, ~80000 columns. I want to split it up and solve subproblems on different machines (horizontal scaling).
But I need to make sure every column can be combined with ...
0
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0
answers
106
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Prove that the total number of parenthesizations of n matrices is Ω(4^n/n^3/2)
Is it possible to prove the total number of parenthesizations of n matrices is Ω(4^n/n^3/2) using the Induction Method?
Recurrence formula from CLRS book
When n = 1, the sequence consists of just one ...
0
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0
answers
240
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Binary Matrix covered with squares to cover all 1's in min cost
So I recently came across a question in my Algorithms class.
Given a binary matrix of N X N.
we can do following operation any number of times
we can take a square of size M X M (1 <= M <= N) ...
1
vote
1
answer
55
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Having a 2D matrix with three typed elements, how to efficiently cover one of the types and NOT cover the other one?
I have a matrix with three possible elements: A, B and C. The size of the matrix could be a maximum of 15x16.
$$
\begin{bmatrix}
A & A & C & A\\
A & C & B & C\\
A & C & ...
- 111
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0
answers
49
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Complexity of calculating Takagi's factorization of $n \times n$ matrix
As described here the Takagi factorization of a square symmetric complex matrix $A=VDV^T$ where D is a real nonnegative diagonal matrix, and V is unitary. I'm wondering what the complexity of ...
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2
votes
1
answer
60
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Finding a map between two matrices that minimises distance differences of neighbors
Given two binary matrices of the same size with the same element counts, how can we find a map mapping ones to ones such that the sum of differences of distances of all pairs of neighbors is minimised,...
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1
answer
40
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Time reversible Markov chains question
Let Q be a symmetric transition probability matrix on states 1 , . . . , n; that is $q_{i,j} = q_{j,i}$ for all i , j.
Consider a markov chain defined as follows. Whenever the chain is in state i, the ...
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39
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Display XY data on computer
I would like to experiment with displaying the output of an analog to digital converter on computer. Samples of the ADC output would determine the intensity of one pixel.
The input to the ADC is an ...
0
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0
answers
66
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Given a 2D Array (of 0's and 1's), find the minimum number of rows required so that maximum columns have their sum greater than a threshold
I have a 2D array of some rows and columns which are having only 0's and 1's. I would want to know if there is a way to optimize the number of rows so that maximum number of columns have their column ...
0
votes
1
answer
60
views
Whether a number is in a sorted matrix
I have a square n-by-n matrix of integers. The number of columns is equal to the number of rows.
Each column and row are sorted from the lowest to highest numbers. For example:
Another example:
I ...
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1
vote
2
answers
392
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Counting the number of parenthesization
I'm reading CLRS and there is something I don't understand regarding counting the number of parenthesization, in the Matrix-chain multiplication chapter, the book says:
Denote the number of ...
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1
vote
1
answer
148
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Complexity of finding $d$ largest eigenvectors of a symmetric matrix
I know that for $n \times n$ matrix, it takes $O(n^2)$ time complexity to compute the largest eigenpair of the matrix using Power method or etc. I'm interested to further extend the case so that now ...
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2
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89
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What kind of algorithm do i need when the place of numbers changing according to n number?
I have a project in Golang. But i don't have any idea about how to solve it.
n will be an odd number (Feedback will be given if an odd number is not written)
As output, a structure with n*n matrix ...
1
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0
answers
23
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Algorithmic ideas to multiply two tall & skinny matrices into one large square matrix?
This problems comes from AI, and it looks something like this:
I am supposed to multiply two floating-point matrices A * B. A ...
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0
answers
19
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runtime of solving matrix differential equation wrt dimensions of matrix
Suppose a computer solves a coupled differential equations (with given boundary conditions) of which each equation deals with $2^n \times 2^n$ size of matrices as solutions. My question is
Does time-...
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3
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0
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161
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Low-rank matrix completion is NP-hard
In looking into the problem of low-rank matrix completion / relaxations of the general problem to derive exact solutions, many papers cite that the original formulation is NP-hard but I cannot find a ...
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1
vote
1
answer
39
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Find indices of equal cells in a matrix
I am trying to find the indices of all the equal elements in a matrix $\left ( n\times m \right )$. For each pair of matching cell, I will perform a specific function on them. For example:
$ \begin{...
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4
votes
1
answer
595
views
Maximum sum of values in a square grid (one in each row/ column)
this is my first post here so bare with me :).
What i'm looking for is an algorithm that can find the maximum sum of values in a square grid under the restriction, that you can only pick 1 value from ...
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1
vote
0
answers
57
views
Time complexity of computing general imminant
Consider the immanent of $n \times n$ complex matrices
\begin{equation}
\operatorname{Imm}_f(A) = \sum_{\sigma \in S_n} f_n(\sigma) A_{1 \sigma(1)} \cdots A_{n \sigma(n)}.
\end{equation}
Here $f_n:\pi ...
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votes
3
answers
160
views
Data structure to efficiently add zero-rows to a sparse matrix
I would like to create a data structure representing a sparse matrix, where the number of non-zero values is $\mathcal{O}(n)$ (the matrix is $n\times n$).
The matrix should support the following ...
- 141
1
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0
answers
194
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Describing the subproblem graph for matrix-chain multiplication
From Introduction to Algorithms by CLRS:
15.2-4 Describe the subproblem graph for matrix-chain multiplication with an input chain of length n. How many vertices does it have? How
many edges does it ...
4
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0
answers
66
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minimizing a pairwise sum with respect to a sequence of integers
Let $m$ and $n$ be two integers, where $m \leq n$.
Suppose you are given $m^2$ matrices $W^{i,j} \in \mathbb{R}^{n \times n}$ for $i, j \in \{1, \dots, m\}$.
The goal is to find a sequence $a$ of $m$ ...
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9
votes
1
answer
641
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Optimizing a sum of matrix chains
Edit Jan 31: important special case is when the sums form a nested structure, search for "Hasse diagram is a tree" below
Here's a practically relevant variation on matrix chain problem:
Find ...
- 201
4
votes
2
answers
114
views
Best-known complexity for $l \times m$ by $m \times n$ matrix multiplication?
I know that the fastest known algorithm for multiplying two $m \times m$ matrices runs in time $m^{\omega}$, where currently we have $\omega = 2.3728596$ due to Virginia Williams's latest result
(see ...
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8
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11
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4k
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Real life examples of *zero* weight edges in graphs
The meaning of edges with zero weight in a weighted graph questions me for a long time, and I even asked a related question previously.
Yet, when I recently read here a question on real life example ...
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2
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0
answers
87
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Is it NP-hard to find different roots of different matrices simultaneously?
Consider the following problem:
input: pairwise distinct natural numbers $k_1,\dots,k_m$ that are all $\leq n$, and matrices $A_1,\dots,A_m \in \Bbb Q^{n \times n}$ where $m \leq n$.
output: a ...
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1
vote
1
answer
179
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A hash function for a 2D hash table with a scattering property?
I have an $n\times n$ matrix, and want to find a bijective function $h:[n^2] \to [n]\times [n]$ that can act as a hash function to map the numbers 1 through $n^2$ to row/column indices in my matrix. ...
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6
votes
1
answer
163
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Computing tr(ABCD...)
Suppose we have $k$ $n\times n$ matrices $A,B,C,\ldots$. Is there a way to compute/approximate the trace of their product much faster than computing/approximating the full matrix product? IE, ...
- 201
2
votes
1
answer
103
views
Cover 2 by n binary matrix with submatrices of minimum total size
This is a homework problem. Let $A$ be an input binary matrix of size $2 \times n$, and $L$ an integer. The objective is to cover all 1s in $A$ with submatrices, such that we minimize the sum of the ...
- 115
3
votes
1
answer
264
views
N points with maximum sum distance
Given a distance matrix for 50,000 points, how do I select $N$ points so that the sum of all distances between the $N$ points is maximized? $N$ could be as high as 100.
To calculate the sum of ...
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