John D'Errico

Retired

Last seen: Today | Active since 2006

Followers: 9 Following: 0

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A retired applied mathematician, consultant, sometime mechanical engineer, numerical analyst, woodworker, bridge player. Please DON'T contact me, as too many students then decide I am their personal consultant if I allow it. I won't do consulting by mail.

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Answered
Remove exponentials with positive argument in symbolic MATLAB
Very often symbolic computations are less easy than you want them to be, because the computer does not see one thing as simple v...

3 dagen ago | 0

Answered
I need to create a polygon or buffer along an irregular shaped coastline in a 2D array of gridded sea temperature data.
Easy enough. Almost trivial, in fact. Of course, making a buffer zone of width exactly one pixel is alays going to be slightly p...

3 dagen ago | 0

Answered
Derive full formula from fitlm(X,y,"quadratic")
X = rand(258,6); y = rand(258,1); mdl = fitlm(X, y, "quadratic") There are 6 variables, here called x1...x6, corresponding ...

3 dagen ago | 1

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Answered
Running a large array
Memory is cheap. Get more memory. I'm sorry, but if you want to work with big data, you will often need sufficient capabilities...

4 dagen ago | 0

Answered
Why did I get two different results in nonlinear programing problems
You have a nonlinear problem. You need to understand that given any set of starting values, an optimizer will sometimes find a s...

4 dagen ago | 0

Answered
The equation of the curve at the intersection of the two 3D surface, One is a cylinder.
It is not always trivial to find that curve as some explicit function. Here, yes, you can do so easily enough. syms x y z r = ...

7 dagen ago | 1

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Answered
creating loop that stops at correct formula for the given input.
You have a fundamental problem here, that you fail to understand. You CANNOT solve for more than 6 parameters, given only 6 data...

7 dagen ago | 0

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Answered
Help solving linear ordinary differential equation that involves matrix
I would postulate that in fact, it DOES give the correct solution, at least to the problem you posed, but that it is not in the ...

10 dagen ago | 0

Answered
How to solve this problem combining sequences and reasoning using code?
I would probably think first, and only then write code. I gather the global sequence will always be 1:(4*m+2), so the first 4*m+...

11 dagen ago | 0

Answered
Find closest value in array
To be honest, the easiest way is to use knnsearch. It works well in one dimension, as you have here, and it should be quite effi...

16 dagen ago | 1

Answered
Calculations with distributions of variables only
Um, yes. And, no. Firstly, very likely there will be no "named" distribution for even such a simple expression. And depending ...

17 dagen ago | 0

Answered
How to fastly calculate this matrix operation
First, use better variables. I is a terrible name to use , especially capital I, since it is so easily confused with the number ...

17 dagen ago | 0

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Answered
Find the turing pattern for following equation
I won't do your homework, since this is highly likely to be homework with no effort made on your part, just from the way you ask...

20 dagen ago | 0

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Answered
In Pskmod Code: Reshaping the output signal using the transpose operator .' , Introduce a sign change.
No. You have it wrong. Completely backwards, in fact. The .' operator does NOT cause a sign flip. That is called a conjugation. ...

20 dagen ago | 0

Answered
Why am I getting gaps in my data when I attempt to interpolate my curve?
As is always the case, look at what you did. Think about it. If necessary, think again. xy = readtable('Sample Test.txt') plot...

21 dagen ago | 2

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Answered
How to call a own function in other mfile?
Did you save it on your search path? (Do you know what your search path is?) Spend some time reading the help for tools like add...

24 dagen ago | 0

Answered
Show Only certain variables in the workspace
The point of what @Stephen23 and I are trying to tell you is that you are asking the wrong question. You want someone to teach y...

25 dagen ago | 0

Answered
Solve a system of two variable inequalities with symbolic toolbox
fimplicit does not plot inequalities. Sorry. If you read the help, it never says anything about inequalities. Simplest is to ju...

25 dagen ago | 0

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Answered
How to fix "Error using indexing" while calculating differential equations?
The code you show will work perfectly in MATLAB, in literally any recent release of MATLAB. R2023 is entirely recent. However, ...

27 dagen ago | 0

Answered
Writing a code to determine the gradient change in a profile
Your problem lies in this line: change_p = (abs(data.variable.gradpressure(i,j+1))-abs(data.variable.gradpressure(i,j)))/abs(da...

29 dagen ago | 0

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Answered
Could MATLAB please provide rref() without the rational conversion?
Just look at the code. type rref Do you see the very last thing it does is do the conversion? Just write your own version that...

29 dagen ago | 0

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Answered
Why does eig return different eigen values depending on whether eigen vector outputs are specified ?
Gosh, the eigenvalue questions are coming fast today. You have an 11x11 matrix. The eigenvalues of that matrix are equivalent to...

29 dagen ago | 0

Answered
How to interprete an analytical eigendecomposition for a polynomial matrix with size nxn > 4x4 using the Symbolic Math Toolbox.
Ok, do you understand that you CANNOT compute the roots of a polynomial, if the degree of that polynomial is greater than 4? Wel...

29 dagen ago | 1

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Answered
Strict inequalities are not supported (learn why)
It is NOT related to the release number. Sorry. That has nothing to do with it. This has been the case since release 0 of MATLAB...

ongeveer een maand ago | 0

Answered
Curve Fitter Polynomial Plots Strangely In Script
CONGRATULATIONS! You win an award for the one millionth person to make this mistake! Your award is a free trip to the fabled vac...

ongeveer een maand ago | 0

Answered
Why is indexing a sparse matrix not behaving how I would expect?
This is not a question about sparse matrices. Well, it is, but it is not really. Sparse matrices are exactly the same as any oth...

ongeveer een maand ago | 0

Answered
how to write the right code to measure the elapsed time of this method d1 = diag(1./diag(A))
Why do you care? The computations will be almost inifinitessimal, even for a huge matrix. (Do you understand the notation O(n)?)...

ongeveer een maand ago | 1

Answered
Alternative to interp2
Perhaps you are calling interp2 multiple times in a loop, when a single call with multiple points will be far faster. This would...

ongeveer een maand ago | 1

Answered
How can people who are bad at math and have no programming aptitude learn MATLAB? (Long question)
An old question, one that surely would be better placed in a discussion, now that we have them. However, as soon as someone dec...

ongeveer een maand ago | 0

Answered
Someone that works with symbolic variables?
When k is even slightly large (apparently 4 is large enough) things go to hell. Did I not answer this question before? I know I ...

ongeveer een maand ago | 1