# Tagged Questions

Questions on the manipulation of matrices in Mathematica.

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### Mapping over two indices with a condition

How can I use Map over two indices with a condition? I am trying to calculate second derivative of an eigenvalue, $\lambda_i(x)$, of $n \times n$ matrix $M(x)$ ...
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### How do I put matrices next to each other? [duplicate]

So I want to be able to put matrices "next to each other", or in other words if I have two matrices $A$ that is $n\times k$ and $B$ that is $n\times m$, I want to be able to create a matrix $C$ that ...
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### Simple code to write differential equations in a matrix formulation

Is there a simple code to transform the following differential system equations : ...
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### Problem with ArrayFlatten to concatenate matrix

I would like to concatenate 2 matrix : - the first which is a line of 2 elements - the second which is a matrix of 2-2 elements I use the following code : ...
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### Count degrees of freedom of a polynomial

I want to count the independent degrees of freedom of a polynomial in three variables $(z_0,z_1,z_2)$. Therefore I take the coefficients of $f$ and compute the rank Jacobian with respect to the ...
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### How to be more efficient/faster with this MatrixExp-vector multiplication?

I am looking for expertise that helps to improve speed of the code below. First, a little bit of background: There is some system of differential equations $\dot{\vec{m}}(t)=S(t)\vec{m}(t)$. In order ...
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### Multivariate Path Construction using Sobol numbers

I am sharing my code,which I have tried to perform as per the instruction mentioned below. Please correct it so that I could get my output. Instruction: Use the first m dimensions of the Sobol vector ...
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### Update or Animate an ArrayPlot or MatrixPlot?

I am trying to add new row or column or points to a 2D plot as I keep generating the values. The trick is by using this method I will not be required to store the big array or matrix and will be ...
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### How to force MatrixPlot to differentiate distinct expr?

I thought MatrixPlot can differentiate different expressions in color until I found this: ...
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### NMinimize error

I want to use NMinimize in the following way: ...
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### How to convert an output of a matrix into a PNG image?

Using @m_goldberg's code I have created a matrix with the following command: ...
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### Sparse Cholesky Decomposition

I am working with square matrices with a special form, which for large rank ($> 100,000$) would be best stored and manipulated as a SparseArray. I believe that ...
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### How to create a general format of a matrix with specified entries?

I want to create a PNG picture of a matrix where its entries are the following: ...
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### Bin data from an imported excel file by adding 5 elemets of y axis each [closed]

So i imported a file from excel and viewed it as a grid. but now i need to add groups of 5 elements from the y axis and plot them against every fifth entry from the x axis. Thanks Edit: Sorry about ...
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### Finding a unitary matrix in Mathematica

I have the following equation : $\tilde{B} = U B U^{\dagger}$ I also know both $B$ and $\tilde{B}$ , I just want to find the matrix U, that gives me the transformation. I tried using ...
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### Speed matrix multiplications

I am trying to run this function ...
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### How do you concatenate matrices? And how do you make block matrices?

This is pretty trivial to do in Matlab, but I have not seen a function for it in Mathematica. Here is how you can do it with Join and ...