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Questions tagged [lu-decomposition]

Questions regarding the numerical method LU decomposition to decompose a matrix into the multiplication of two triangular matrices: A lower triangle matrix and an upper triangular matrix

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Binomial determinant and LU decomposition

Let $A_n$ be following $n \times n$ symmetric pentadiagonal matrix $$ \begin{pmatrix} 6&4&1&& \\ 4&\ddots&\ddots&\ddots&\\ 1&\ddots&\ddots&\ddots&1\\ &...
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linear algebra - prove that a matrix is positive definite [duplicate]

I'm given a matrix $K$ that is symmetric and positive definite. I'm asked to prove that $K_2 - (l\times u)$ is also a positive definite matrix. $K_2$ is a submatrix of $K$ so that we get rid of the ...
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Shortcut method for LU decomposition method so that A=LU

To prove that if matrix $U$ is obtained from $A$ by the row operation $R_i = R_i + kR_j$, then $L$ is obtained from the identity matrix $I_n$ by the reverse row operation $R_i = R_i - kR_j$, such that ...
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Proof that the inverse of a nonsingular lower triangular matrix is also lower triangular [duplicate]

Is anyone able to construct an easy-to-follow proof that the inverse of a nonsingular lower triangular matrix is also lower triangular? I have been attempting to construct a proof by contradiction to ...
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Decompositions of symplectic matrices over the integers

Given a symplectic matrix $S \in \text{Sp}(2n,\mathbb Z)$ whereby $S^T\Omega S=\Omega$ with $$\Omega=\left(\begin{matrix}0&I_n\\-I_n&0\end{matrix}\right)$$ what known decompositions exist such ...
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Counting LU decompositions of 2 by 2 matrices over a finite field of 5 elements.

Let $G=GL(2, \mathbb{F}_5)$, i.e., group of invertible $2\times 2$ matrices over a finite field with $5$ elements. Let $S$ be the subset of those elements of $G$ that can be written in the form $LU$, ...
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Find the $LU$ decomposition of a Laplacian like matrix.

Question: We have a matrix defines as follow: $\underline{\underline{A_n}} = \begin{pmatrix} 1 & -1 & 0 & ... &0 \\ -1 & 2 & -1 & ...&0\\ 0 & -1 &...
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Show that the elements of the LR decomposition for tridiagonal matrices T can be determined by the following recursive relation

Show that the elements of the LR decomposition $ T=L R $ can be expressed by tridiagonal matrices $ T $ with can be determined by the following recursive relation: How could you prove this statement ...
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Banded Matrix LU Decomposition

I have an $n$ by $n$ $k$-banded matrix for which I calculated the LU decomposition via Matlab. Now, I want to solve the system to find the resulting vector and compare the operation count with another ...
Cooltafel's user avatar
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Is the LU decomposition just Gauss-Jordan elimination?

I am watching Gilbert Strang's neat lecture on the LU decomposition, which is taught just after Gaussian elimination. $LU$ for a matrix $A$ was found doing $EA=U$ and finally $A=E^{-1}U$. Seems to me, ...
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Missing the point of LU factorization / decomposition

Gaussian Elimination The system of linear equations $Ax = b$ may be solved by using Gaussian Elimination (GE) arriving to a Row Echelon Form R of the augmented matrix $[A b]$, and then using back-...
Mah Neh's user avatar
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Solving linear system with product of two N by N matrices

I'm trying to find an efficient way to solve a system of equations given by $STx = b$ where $S, T$ are N by N matrices that have a given LU decomposition for each. I believe the most efficient ...
NewToThis's user avatar
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Prove that a given statement is equivalent to a matrix possessing an $LU$-factorization

An $LU$-factorization of a matrix $A$ is a way to write $A$ as the product of a lower triangular matrix $L$ and an upper triangular matrix $U$, where the lower triangular matrix has $1$'s on its ...
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Uniqueness of the LUP decomposition

Given the matrix $$A=\begin{pmatrix} 0 & 0 & 2 \\ 1 & 2 & 3 \\ 2 & 2 & 3 \end{pmatrix},$$ I want to compute the LUP decomposition $PA=LU$. Here's what I got: $$P=\begin{pmatrix}...
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Can a matrix not admitting an LU decomposition be shifted so that it does admit an LU decomposition?

Almost all square matrices have an (unpivoted) LU decomposition, but some don't. The question I have is whether - assuming $M$ is a square matrix over $\mathbb R$ or $\mathbb C$ which does not admit ...
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Does $M^2$ have an LU decomposition?

It's well known that matrices of the form $M^T M$ have an (unpivoted, of course) LU decomposition. In fact, because they are positive semidefinite, they have a Cholesky decomposition. The field here ...
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Prove case for why LU Decomposition fails

I understand that for some square matrices, a LU decomposition (not talking about LUP) can fail to exist. For example, the following matrix has no LU decomposition $$ \begin{bmatrix} 0 & 1 \\ 2 &...
esnoopy808's user avatar
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Find the volume of the box in $R^3$ that is spanned by the columns of $A$.

The full question is Consider the matrix $A=\begin{bmatrix}1&-1&1\\1&1&1\\1&-2&2\end{bmatrix}$. (a). Find the $LU$ decomposition of $A$. (b). Find the volume of the box in $R^...
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Decomposing a matrix that has duplicate columns using PA=LU factorization

I am given the following matrix. $$ A = \begin{bmatrix} 3 & 3 & 9 & 6 \\ 4 & 4 &4 &4 \\ 1 & 1 & 5 & 5 \\ 2 & 2 & 4 & 6\end{bmatrix} $$ As you can notice,...
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Ways of showing that the LU decomposition of a square matrix almost always exists?

We know that (in the measure-theoretic sense) almost all square matrices $M$ over $\mathbb R$ admit an LU decomposition: $M = LDU$. We are using the Lebesgue measure, and we may treat for each fixed $...
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Norm of Lower Traingular matrix of a LU decomposition $\|L\|_{\infty}\le n $

In LU factorization of a square matrix , show that $\|L\|_F \le \frac{n(n+1)}{2}$ $\|L\|_{\infty}\le n $ $\|L\|_1\le n$ $\|L\|_2 \le n$ Now here in $L$ all diagonal elements are $1$.Now Frobenius ...
Aritra Roy's user avatar
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Completing the square when the pivot are zeros.

I watched the 18.06 MIT course on linear Algebra online. It shows a nice method to complete the square using Gauss elimination. For example if I consider the quadratic form \begin{equation}x^2+y^2+4xy=...
lambertmular's user avatar
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Do positive semidefinite matrix have LU decomposition? [closed]

Suppose we have a real matrix $A=R R^t$ where $R$ is triangular. Since there is no restriction on $R$ we can say that $A$ is positive semidefinite. Can I affirm that $A$ has LU decomposition (L lower ...
nostromo9's user avatar
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Matlab. LU decomposition Crout's Method. And usage of decomposition on accurate Block Matrix.

I have to implement such a program()Look at picture I attached I mostly implemented everything: Crout's Algorithm, solving linear equations, I created this block matrix, but I don't know to use that ...
Anforetta Langdon's user avatar
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MATLAB Code Help. Using Crout's Method, solve the system of linear equation $Mz=f$, where $M=\begin{pmatrix}I &A\\A^T&0\end{pmatrix}$

Using Crout's Method, solve the system of linear equation $Mz=f$, where $$M=\begin{pmatrix}I &A\\A^T&0\end{pmatrix}$$ I have implemented algorithm of Crout's method. But I don't have any idea ...
Anforetta Langdon's user avatar
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What's the fastest way to solve a system of equations a million times such that the coefficient matrix is same but the constant matrix is different?

I need an efficient way to solve $Ax=C$ a million times such that coefficient matrix $A$ is always the same but the constant matrix $C$ is always different for each of the million problems. To solve ...
ishandutta2007's user avatar
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Why $L^{-1}U^T=D$ in LU decomposition?

I learned that given a matrix $A$, we can apply LU decomposition to get $A=LU$, where $L$ is lower triangular and $U$ is upper triangular. Further, if $A$ is symmetric (or Hermitian for complex $A$), ...
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Determine if LU decomposition is possible on a matrix?

I am trying to understand how you determine if LU decomposition is possible on a given matrix. I believe the way to calculate this is to check if the leading-matrices have non-zero determinants. I ...
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Do the rows used in row operations during LU factorisation matter?

A method I have seen for finding the LU factorisation of a matrix is that U is the row echelon form of A. The row operations we perform on A to get to U must involve replacing $R_i$ by $R_i - kR_j$ ...
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Solving System of linear equation using LU decomposition

I am working on a simultaneous linear equation problem using LU decomposition and I'm unsure if this is the correct approach/answer to solve a system of simultaneous equations using LU decomposition. ...
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Solving System of linear equation containing decimals

I am working on a simultaneous linear equation problem using LU decomposition. $5x1 + 6x2 + 2.3x3 + 6x4 = 4$ $9x1 + 2x2 + 3.5x3 + 7x4 = 5$ $3.5x1 + 6x2 + 2x3 + 3x4 = 6.7$ $1.5x1 + 2x2 + 1.5x3 + 6x4 = ...
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Is my LU Factorization Incorrect?

I have the following matrix I am trying to decompose into it's respective $L$ and $U$ parts for $A = LU$. So I have $$\begin{bmatrix} 1 && 4 && 3 \\ 0 && -10 && -5 \\ 0 ...
sekib72409's user avatar
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generalization of LU decomposition?

I've just begun studying numerical approaches to LU decomposition and it got me thinking. Is there a more "general" (not sure if this is the right term for what I'm describing) form of LU ...
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LU- decomposition

Find an LU-decomposition of the coefficient matrix and solve the system $$\begin{pmatrix} 1 & 4 & 3\\ -1 & -1 & 3 \\ 2 & 9 & 8 \end{pmatrix} \begin{pmatrix} x_1 \\ x_2 \\ x_3 \...
102math's user avatar
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A question about the product of commutators in an article of Vaserstein

The question comes from Lemma 13. It is stated as follows. Let $A$ be an associative ring with $1$, and $n\geq2$ an integer. Assume that either $n\geq3$ or $n=2$, and $1$ is the sum of two units in $...
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Necessary and suffcient condition for LR-decompositions

Let $ A=\left(a_{i j}\right)_{i, j=1, \ldots, n} \in \mathbb{R}^{n \times n} $ be an invertible matrix. Show that an $ L R $-decomposition of $ A $ exists if and only if $ \operatorname{det}\left(A^{[...
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LU Decomposition without Gaussian Elimination

I'm creating a program to compute the LU factorization of a matrix, and I was wondering if there was a way to compute an LU factorization without using Gaussian Elimination. I'm mainly worried about ...
Faraday's user avatar
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Block LU factorization with more than two blocks?

If I have a symmetric, positive definite block matrix there exists the following LU decomposition: $$\left[\matrix{A && B^\intercal \\ B && C}\right]=\left[\matrix{A^{\frac{1}{2}} &...
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How to find a solution other than the vector $0$ of Linear System $AX=b$ with $b$ belonging to $0$

Hello I am currently trying to find the solution of a spring system without condition on border, so naturally my vector $b \in 0_{M\{n,1\}}$, I am resolving this system with an LU algorithm but ...
Benzait Sofiane's user avatar
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Do you have to start with the first row during LU decomposition?

I guess I've never run into this pretty simple problem, but I'm doing a Cholesky Factorization of the matrix shown below, and ran into some weird ambiguity in how I usually do $LU$ decomp: $$ A = \...
fatngreasy's user avatar
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Efficient LU decomposition of matrix after updating diagonal

I am computing LU decomposition of $(kD + A)$ where $D$ is diagonal matrix with {$d_{1}$, $d_{2}$, ... , $d_{n}$}, $A$ is a real symmetric positive-definite matrix, $k$ is a number that changes on ...
dimaMS's user avatar
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Is there any situation where the LDU decomposition is the same as the eigenvalue decomposition?

I was just wondering if there are any situation where the LDU decomposition is the same as eigenvalue decomposition (diagonalization)? The only way this can be possible if L and U are inverse so ...
Bill's user avatar
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DU decomposition

My professor said that we can't express a square matrix as a product of upper triangular matrix and lower triangular although it can be expressed as a product of lower triangle matrix and an upper ...
Neeraj Chauhan's user avatar
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Example of regular tridiagonal matrix $A$ with given properties

I am looking for a regular tridiagonal matrix $A$ such that at the LU-decomposition with partial column pivoting the matrices $L$ and $U$ are also tridiagonal, but with total pivoting the matrices $L$ ...
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Calculate the LU-decomposition $PA=LU$ : How do we calulate $L$?

Calculate the LU-decomposition $PA=LU$ for the matrix $$A=\begin{pmatrix}3 & 1 & -3 & 2 \\ -2 & 1 & 0 & 0 \\ 2 & -2 & 4 & 1 \\ 0 & -1 & -1 & 3\end{...
Mary Star's user avatar
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Understanding the proof the iterative improvement method for the result of linear equation solving using LU decomposition in numerical recipes

The question is from section 2.5 Iterative Improvement of a Solution to Linear Equations in Numerical Recipes book. When we solve $\mathbf{Ax = b}$ using LU decomposition numerically, the result is ...
austin1511's user avatar
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1 answer
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Proving by induction that $L = I + \sum_{ k=1}^{n-1} u ^{(k)} e ^T _k $

Let $L = \prod_{k=1}^{n-1}(I + u^{(k)}e^T_k) $ where $u^{(k)} (i) = 0$ for $i = 1 : k$. Prove that $L = I + \sum_{k=1}^{n-1}u^{(k)} e^T_k$ by induction. (where $L$ is the lower triangular matrix of ...
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Unit lower triangular matrices multiplication

We know that product of two unit lower triangular matrices is a unit lower triangular matrix. However, if product of two lower triangular matrices is unit lower triangular then is it necessary for the ...
Arup Das's user avatar
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Counter example of LU decomposition uniqueness

LU decomposition Theorem: If $A \in \mathbb{R}^{n \times n}$ is such that each principal minor $A_k$ has $det(A_k) \neq 0, \, k = 1, 2, \dots, n-1$, then $A = LU$, beeing $L$ a lower triangular unit ...
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How to switch rows in matrix L when decomposing matrix A into PA = LU?

Find the permutation matrix $P$, the lower triangular matrix $L$ and the upper triangular matrix $U$ such that $$ PA=LU $$ Given $$ A= \begin{pmatrix} 1 & 2 & 3 & 4 & 5 & 6 \\ -2 ...
Operator's user avatar