# Tagged Questions

Questions on the various algorithms used in linear algebra computations (matrix computations).

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### What comes after sorting eigenvalues in PCA?

I'm a student, I have to build PCA from scratch using Matlab on iris data. Iris data have 4 features, I want to reduce them to 2. I reached the sorting of eigenvalues step. What is the next step?
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### Linear Algebra Complex

A homogonous linear system is given by: x1 + x2 = 0 a · x2 + x3 = 0 2·x1 + x2 + a·x3 =0 where a ∈ C, a) Find the determinant of A and give the values of a for which matrix A is ...
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### How to have a consistent or inconsistent linear algebra equation?

I have been a bit confused about this linear algebra question, if someone can explain it would be great. So my professor is asking us to determine all values of x for which the linear system a, is ...
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### partition of block matrices

If $A=\begin{bmatrix} A_{11} & A_{12} \\ A_{21} & A_{22} \\ \end{bmatrix}$ is a partition of $A$ such that $A_{11}$ and $A_{22}$ are $r × r$ and $(n − r) × (n − r)$ ...
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### convergnece of QR-method

I'm studying the QR-algorithm. In particular I have this algorithm: For every $k=0..n$ select a shift $\sigma_k$ factorize $A_k-\sigma_k I =Q_kR_k$ multiply $A_{k+1}=R_k Q_k+\sigma_kI$ muliply ...
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### GAXPY Operations

Let A ∈$R^2$, x ∈ $R^k$. Find the first column of M = (A − x1I)(A − x2I)...(A − xkI) using a sequence of GAXPY’s operations. GAXPY: General matrix A multiplied by a vector X plus a vector Y. I tried ...
I can't understand and even can't find any text on Compression of a matrix A by V. meaning if $B=V^*AV$ then B is called the compression of A. What does it mean???