# 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: 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...

9 hours ago | 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...

10 hours ago | 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...

2 days ago | 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...

5 days ago | 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 ...

5 days ago | 0

vector output of a symbolic vector

syms s X Y Z l1 = X*s - 1 == 3*X + Y + Z + 2/s l2 =Y*s == - X - Y - 1/s^2 l3 =Z*s - 2 == X - Z - (s - 1)/s^2 [Xsol,Ysol,Zsol...

6 days ago | 0

Optimisation of three function in two variables

There may be an issue here, if you think you can maximize them all simultaneously. The (x,y) coordinates that maximizes one func...

7 days ago | 1

About fitting one curve with one equation and calculating two values from fitting

Suppose I gave you a set of data, and asked you to tell me two parameters from the data. All I need is one data point, More woul...

8 days ago | 0

How to fit this model with a Weibull distribution?

If this is data that you think comes from a Weibull, then you do not want to use regression techniques to fit the distribution. ...

11 days ago | 0

how i can show in matlab tha sqrt(x^2)=x in symbolic toolbox

Except, that equality does NOT hold! The sqrt "function" has two branches. sqrt(x^2) can as easily be -x, as it is x. So you can...

17 days ago | 1

I keep getting 4 answers per variable in 4x1 sym

L = 225; % Length of the ship in meters B = 32; % Beam of the ship in meters T = 12; % Draft of the ship in meters v = 0.5; %...

18 days ago | 1

Find zero of simple monotonic function where the independent variable can only be integer

@Michael Albert No. You just THINK there is some obscure MATLAB function to solve your problem. You do not know that to be the ...

18 days ago | 0

Why is my test accuracy higher than validation accuracy?

Not really. It might be a reflection that you needed more data, that your sets are just not large enough. The law of large numbe...

18 days ago | 0

What is happening with the line of best fit here?

StrainStress = [0.00005 010.1; 0.00007 014.3; 0.00011 021.8; 0.00014 027.6; 0.00020 040.0; 0.00030 061....

20 days ago | 1

| accepted

Generating a matrix of 0s and 1s with constrained domains

Are you saying you want to break the matrix into a set of regions, where within the region, the matrix contains zeros, and the r...

21 days ago | 0

How to provide a positive and negative value for the tolerance value?

Good answer by @Voss of course. I'll merely add a few points of interest. You could just test to see if the current term being ...

21 days ago | 1

Finding out lengths of sequences of numbers in a set of vectors

It took a few seconds staring at the output you expect to see, and realize what you meant. I THINK the first element in each row...

21 days ago | 1

Is there a premade file regarding a verification test to see if there is any issues with the installation?

Is there anything that will test every line of code in MATLAB? Every toolbox you have purchased? No. Not that is given out to yo...

22 days ago | 0

I have a question relate to Gauss seidel method

Updating the vector x (small x) and then looking to see if the vector X (capital X) is converging seems like a bad idea. You did...

23 days ago | 0

integral using "int" function

Not everything you write down will have a simple, algebraic solution. As much as people seem to think they are, computers are no...

23 days ago | 0

f(x) = x^2/3(3-x^2)(x-4). How do I enter this function in MATLAB?

How do you enter it? You already did exactly that, at least in symbolic form. syms x real % f= input('Enter the function f(x):...

24 days ago | 0

Very large output numbers

syms Kc s X = (6982830709666455*Kc*s)/140737488355328 vpa(X,10) So here, 10 digits shown.

25 days ago | 0

atan2 does not accept complex numbers as input

It seems the answers provided so far are missing the point. If we want to see a 4 quadrant atan2 version, that works for complex...

26 days ago | 1

| accepted

Generalised solution to a n-order system of equations

(Edited to fix my error in extrapolating your equation system. I had not seen at first you were essentially adding two terms to ...

26 days ago | 2

I need to write a code to find numbers divisible by 7 and multiples of 5 from 1500:2700. How do I preallocate line 4 for speed and is my code the most efficient way to solve

Lol. A huge amount of traffic on this from last night when I looked in. But you did write code. So how would I solve the problem...

27 days ago | 0

Best computer specifications for fast Matlab numerical simulations/integrations?

Something I've said before, when this same question gets asked... If you will do heavy computations for lengthy periods of time...

27 days ago | 1

Patenting a Matlab script

Adding to what @Steven Lord has said, it is not that easy to do. And it is may even be the wrong path to take too. (Even though ...

27 days ago | 0

Why is it when I am adding a number to array, the single number overrides array

E=10.3*10^6; %psi Fty=68000; %psi Ftu=76000; %psi fnl=linspace(0,Ftu,250); e0=0.09; epu=e0-(Ftu/E); nt=log(epu/.002)/log(F...

28 days ago | 1

How to solve for the maximum or minimum value of a symbolic function

You need to understand that not ALL equations you can write down have an analytical solution. In fact, it is trivial to formulat...

1 month ago | 0

why use 0.008856 in Lab color space?

Its been a million years since I looked at those equations. But it seems likely to be almost trivial. They chose a point where t...

1 month ago | 2