# 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.

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**Feeds**

Warning: Equation is badly conditioned. Remove repeated data points or try centering and scaling.

Do NOT use polynomials to model data like this In fact, do not use high order polynomials, pretty much ever. FR = load('FR.txt'...

circa 21 ore fa | 2

Recursively reversing large vector efficiently

An old question with an already good answer. Regardless, it explicitly asks about recursive schemes to flip a vector. And I can ...

2 giorni fa | 0

Fixing biased random number generation

What is the goal of this? That is a good question, as asked by @埃博拉酱. You should recognize that the result of this operation w...

3 giorni fa | 0

My end value "tc2" comes out as "NaN + NaNi", in the following script and I cant figure out why.

A NaN means you are doing some operation that has no meaningful result. For example, what is 0/0? 0/0 A mathematician will tel...

4 giorni fa | 0

Sparse matrix memory understanding

I think you missed a zero. By the way, 5% sparse is NOT at all what I would consider sparse, or for there to often be a gain. L...

7 giorni fa | 0

Fittype and fit do not work as expected (in a curve fitting)

x = [1 4 9 15 25 35 45 55 65 75 85 95 150 250 350 450 900]; y = [0 335 28 37 9 4 3.5 2 1 0.7 0.6 0.5 0.3 0.1 0.01 0.005 0.001];...

7 giorni fa | 2

| accettato

Newton Rapson method code question

Actually, this is surprisingly nice looking code. (A true compliment from me, given most of the student codes we see on here!) Y...

11 giorni fa | 1

| accettato

Converting an STL file to a 3D Surface/Equation

Um, I think you misunderstand a few things. It seems the STL model actually has thickness. So you have exported the entire thi...

11 giorni fa | 0

I want the genetic algorithm to begin with intial parameter and this is not happened can you help me for making the genetic algorithm begin with the intial parameter?

I think you don't understand GA. It does not use a start point. It starts with an initial population of points from the sample s...

12 giorni fa | 0

How do I convert a symbolic expression to a string?

Simple enough. Note the changes I made to your last line only. I could also have done it using character vectors, but then I wou...

13 giorni fa | 1

| accettato

Plotting a volume in 3D

Your question is somewhat incomplete, as to what you mean by plotting a solid volume. I'll assume you may mean something like th...

14 giorni fa | 0

How to numerically calculate volume under the curve?

You have a surface. (Not a curve as I might say.) How do you compute the area under that surface? Simple. Just call trapz twice....

14 giorni fa | 1

| accettato

ode45 integration issues for some choices of physical parameters,

This is an absolutely classic case for a stiff system, where for some sets of system parmeters, you have two or more things happ...

16 giorni fa | 0

How to choose 60 unique coordinates ('A') from 600 coordinates('B'), nearest to 60 other coordinates('C')

As I pointed out to @Voss and @Star Strider, if the points in B mst be unique, then there is an issue. If you simply choose the ...

16 giorni fa | 1

finding variable of a given equation

gamma is a really bad choice of variable names, since gamma is itself a very useful function in MATLAB. Try to avoid doing these...

18 giorni fa | 1

solving nonlinear equation using newton method

You just misunderstand how to use the exponential function. In MATLAB, e^x is written as exp(x). That makes your fn as: fn = @...

19 giorni fa | 0

How to Set a Minimum Step Size for Variables in GA Solver (Global Optimization Toolbox)?

Really easy. Don't think of a step size for the GA solver. It does not take steps anyway, not like fmincon or other solvers of ...

20 giorni fa | 1

| accettato

Solution of system of nonlinear equations

This will be literally impossible to do in double precision, given the huge dynamic range of those coefficients. But we can look...

20 giorni fa | 0

Support with interpolation using interp3 in MATLAB

And ... your question about MATLAB is? Far too much vague verbiage to know what is the problem. If you want to know how to use i...

23 giorni fa | 0

Ode45 g and dg

You cannot use the variable dg4, before it is defined. And we see you do this: dg2=m*A+k*g(1)+h1*k*g(3)+dg4 ... dg4=h1*k2x*g(...

24 giorni fa | 1

fitting with custom equation

(I assume you intended to write 1/y, and not I/y, where the variable I is not defined.) y = [1.6E5 2.5E5 4.1E5 8E5 1E6 2E6 7E6 ...

25 giorni fa | 1

improve fit compared to R

x = [0.858734025700000 0.975248753121875 1.02525125312812 1.13140821289062 1.15927407430000 1.18768630564687 1.21665290240...

26 giorni fa | 1

Some .jpeg photos can not be processed by Matlab code.

I've not even looked at your code. But you have jpg images, uint8 code values. So I wonder if adding the code values caused an o...

26 giorni fa | 0

Finding concave hull around abstract shape

I don't know the soure of this data. But, can you start with the two pieces as separate entities, form polyshapes from each, and...

27 giorni fa | 1

Numerical Integration MATLAB vs Python/VBA

This is difficult to say, because we know virtually nothing. First, does the problem arise from the use of subtly different da...

28 giorni fa | 0

Warning: Explicit solution could not be found./ empty sym

Does an analytical solution exist for EVERY equation you can write down? (No.) Even if such a solution does exist, is it certain...

29 giorni fa | 0

fmincon is not converge

It is very rarely easy to know why a solver fails to converge on a messy thing like this. But you should never let it come to th...

29 giorni fa | 0

Is there any way to perform an at least partially symbolic smulation (using ODE or discrete model) in simulink?

Essentially, no. This is a frequent question about tools like ODE45 too. Simulink is a purely numerical tool, like ODE45 and oth...

circa un mese fa | 0

d solve command matlab

syms e P S A t m V0 t0 V(t) real The point being that assumptions can apply only to variables, not functions. V(t) is a functio...

circa un mese fa | 0

Plotting data dependent on three independent variables.

Do you have a 4-dimensional holographic monitor? That is, you can plot things in 3-dimensions. You will see a projection into a ...

circa un mese fa | 0