John D'Errico

Retired

Last seen: Today | Active since 2006

Followers: 23 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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How to get consistant results when testing code performance
A single time test will be susceptible to anything that attracts the attention of your CPU. And sadly, your CPU suffers from com...

ongeveer 3 uur ago | 0

Answered
update guess in fsolve
Um, no, You cannot always find a way to "update" your initial guess for fsolve. One can even prove this to be true. You can cha...

1 dag ago | 1

Answered
How to remove columns in very large matrices.
GET MORE RAM!!!!!!!!! RAM IS CHEAP!!!!!!!! Need I say it again? You have a quite large array. (1.75e6*2000*8)/1e9 That arra...

2 dagen ago | 1

Answered
using sparse matrix and vpa function simultaneously
I wanted a nice bright red Lambo for Christmas. Nope. Did not happen, nor was it ever going to happen. The same applies to you. ...

2 dagen ago | 0

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Answered
Matrix is singular to working precision
I don't even need to run your code, to see an obvious mistake. In fact, I knew what I expect to be a problem, even before I look...

5 dagen ago | 0

Answered
real and integer variables
No. We won't do what is surely your homework assignment. But why not TRY IT! First, plotting it is trivial. LEARN TO USE a tool...

12 dagen ago | 0

Answered
Why matlab is not as intelligent as python
Why? Why is anything the case? Because it is. Because they are two different languages, with different capabilities, and differe...

14 dagen ago | 3

Answered
Matlab sparse matrix does not come out as I would expect
S=sparse([1 3 2 1 4],[3 1 4 1 4],[1 2 3 4 5],4,4) full(S) It works just fine. What did you do wrong? LOOK CAREFULLY AT WHAT Y...

14 dagen ago | 1

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Answered
defining upper and lower limits of a matrix
Is there any way to set the min and/or maximum values of the elements in a matrix, and do so automatically? NO. Well, not withou...

20 dagen ago | 1

Answered
how to optimize block toeplitz matrix ?
The one thing you NEVER, EVER, EVER want to do is stuff the elements into a sparse array iteratively. NEVER DO THAT!!!!!!!!!!!!!...

21 dagen ago | 0

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Answered
PC specs for heavy & complex simulations
No computer will ever be as fast as you want it to be. Even if you do get the fastest, most powerful computer you can possibly ...

22 dagen ago | 0

Answered
Which statistic is minimized in curve fitting app
The sum of squares of the residuals is mimimized. SSE effectively stands for Sum of Squares of Errors. What is the 2-norm? Just...

22 dagen ago | 1

Answered
I need to draw a shape in matlab help me
This is not a difficult problem. Not at all. But it is impossible for us to do it, based on having been given no information exc...

22 dagen ago | 0

Answered
Simmetrical Constraints in Multivariate Regression
Now please stop asking this question. I already suggested 3 ways you MIGHT solve it, the first time you asked. You could have ad...

23 dagen ago | 0

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Answered
Find limits satisfying a condition on an integral
You have posed a interesting problem, with a subtle flaw that possibly you don't understand. You asked to find an integral limi...

24 dagen ago | 0

Answered
Simmetry constraints in multivariate regression.
Sure. In fact, I can think of at least 3 ways to do so, almost trivially. Just estimate the upper (or lower half) of the matrix...

24 dagen ago | 1

Answered
how to find curve fiitng equation through custom equation
You can't. Yes, I want world peace. But do you think that just wanting it is sufficient? Nothing I do will change it either. The...

26 dagen ago | 0

Answered
Accidental deletion of project, how to contact file exchange admin
Why did you do that? Silly question, I know. ;-) Yes, it should be undoable. I'll contact the FEX admin and ask them to resolve...

26 dagen ago | 0

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Answered
Complex multiplication giving incorrect result
Yet another person who does not understand the realities of double precision arithmetic. Note that the imaginary part is roughl...

ongeveer een maand ago | 2

Answered
What is the best kind of MATLAB function to teach beginners?
I think I agree with @Stephen23, in that teaching symbolics first can cause issues. We see many new users who think everything s...

ongeveer een maand ago | 1

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Answered
Different results of the same caculation by loop code and by array code
As is almost always the case, when we see this complaint, it is because you do not understand how arithmetic works in terms of d...

ongeveer een maand ago | 0

Answered
我买了学生版
Are you sure you have the student edition, and not the 30-day free trial? It would appear you have gotten the trial version, by ...

ongeveer een maand ago | 0

Answered
MINLP problem in MATLAB with both integer constraints and nonlinear equality constraints
NOPE. Were I writing that code, hoping to solve a problem with integer variables, and general nonlinear equality constraints, et...

ongeveer een maand ago | 1

Answered
How to stabilize the edge effects in a cubic B-spline?
You don't show any data, so there is little we can actually do in a constructive way. But you don't have a great deal of control...

ongeveer een maand ago | 0

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Answered
What is the origin of "Weissinger Implicit ODE"
I usually laugh at the results of the AI tools available. Half the time they are blowing smoke, since they don't really understa...

ongeveer 2 maanden ago | 0

Answered
my code does not have an answer
The failure of both vpasolve and fsolve suggests it is lkely no solutions exist at all for your problem. But merely the failure ...

ongeveer 2 maanden ago | 0

Answered
Improved Euler's method compared to 4th order Runge - Kutta's method
Is it as good? COMPARE IT TO THE ANALYTICAL SOLUTION! We have gone through exactly this before. A big problem however, is you a...

ongeveer 2 maanden ago | 0

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Answered
Clarification on Calculations for $v_{1,z} $ in "A Method for Verifying the Generalized Riemann Hypothesis"
I think, from almost line 1 of what you did, you don't understand double precision arithmetic, and that will cause everything yo...

ongeveer 2 maanden ago | 0

Answered
Is the methodology used in "sdRemoveImpossibles" a heuristic or is it derived from an academic publication?
Note that just because something is found in a paper, does not make it even necessarily good mathematics. The lack of any requir...

ongeveer 2 maanden ago | 0

Answered
Infinite while loop for Euler's method in order to solve ode
I think you misunderstand what is happening. Your ODE is y' = y - y^2/2 ymax is found when y' = 0, and so you find that y...

ongeveer 2 maanden ago | 0

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John D'Errico

Retired

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