# John D'Errico

### Retired

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.

**Feeds**

How to choose a single element randomly from a vector

I've upvoted the answer by @R. Please accept it. My answer here is only to express some ideas that are really best put in a comm...

1일 전 | 0

Setting unknown constants in equation for known data

This is the classic regression problem, either linear or nonlinear, and that depends on how the parameters enter into the "model...

4일 전 | 1

Efficient management of interacting atoms based on their mutual distance

No. It is NOT true that in a set, if atom x is one of the k closest atoms to atom y, that the converse is also true, i.e., that ...

4일 전 | 0

Numerical integration of area

The region in color? The integral will be negative, or course, since depth is entirely negative. But you perform such an integr...

5일 전 | 1

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Particle Swarm Optimization - What method is used to find local minimum?

Particle swarm optimization, https://en.wikipedia.org/wiki/Particle_swarm_optimization is one of many meta-heuristics used to ...

5일 전 | 0

Any ideas for making this exponential decay function match the actual data better?

(I spent so much time writing this as a comment, I might as well make it an answer.) @Image Analyst asks a valid question. I ha...

5일 전 | 1

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...

11일 전 | 0

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...

12일 전 | 0

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 ...

12일 전 | 1

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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...

13일 전 | 0

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...

13일 전 | 0

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 = ...

15일 전 | 1

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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...

16일 전 | 0

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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 ...

19일 전 | 0

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+...

20일 전 | 0

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...

25일 전 | 1

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 ...

26일 전 | 0

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 ...

26일 전 | 0

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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...

28일 전 | 0

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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. ...

28일 전 | 0

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...

30일 전 | 2

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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...

대략 1개월 전 | 0

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...

대략 1개월 전 | 0

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...

대략 1개월 전 | 0

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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, ...

대략 1개월 전 | 0

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...

대략 1개월 전 | 0

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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...

대략 1개월 전 | 0

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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...

대략 1개월 전 | 0

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...

대략 1개월 전 | 1

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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...

대략 1개월 전 | 0