# John D'Errico

### Retired

DON'T contact me. I won't answer questions. PERIOD. I don't do your homework. I won't do consulting. A retired mathematician, numerical analyst, woodworker, bridge player.

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**Content Feed**

How to solve a large 6-variable non-linear matrix determinate equation for all unique values.

This is an almost impossible problem to solve symbolically. Why? symbolic computations using determinants become INCREDIBLY comp...

1 day ago | 0

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vpa is not outputing enough digits

I think you may have a setting messed up on your computer. For example... X = sym(3)/32; vpa(X) ans = 0.09375 But now, if I...

1 day ago | 0

change of expression ?

help solve help isolate Either will work. Why not try them? Best would be to read the documentation, the tutorials. You would ...

1 day ago | 0

Matlab and Fortran Precision Issue

No. In the most common case, such a disagreement is because you copied over some numbers from Fortran to MATLAB (or the other w...

1 day ago | 1

Starting Code from Middle

Why? Because you are not doing it correctly? What can I say? Maybe you are just running a really old version of MATLAB. Most lik...

3 days ago | 0

Difference between fittype 'poly1' and 'A*x+B'?

When you use the 'poly1' model, FIT is probably smart enough to understand this is a LINEAR model. And that it is solvable using...

3 days ago | 0

fitdist should be pdf^-1 but is seems not to return correctly

The problem is, you do not understand that evaluating the PDF is NOT the same thing as sampling random numbers from that distrib...

3 days ago | 0

| accepted

How to solve 4 non linear equations with 5 unknowns using least square method

You have an under-determined problem. If any solution exists, then there will be infinitely many solutions. (This is the case in...

3 days ago | 0

How to solve differential equations with varying constants

If a constant varies with time, then is it really a constant? Of course not. X=[ -47.9440 19.0518 -14.4223 -47.9833] If th...

4 days ago | 0

Need matrix or vector with 1,0,1,0,1,0.........

Hint: Use mod. For example, mod(1:25,2) But really, you need to start by looking at the Onramp tutorials, or read any text. I...

4 days ago | 0

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3D plot of an ellipsoid

theta_ = [1 2 3]; COV = cov(rand(10,3)); % picking some arbitrary values for theta_ and COV... RC=@(x,y,z) (theta_-[x y z])*in...

4 days ago | 1

| accepted

Setting a lower bound on the values of an array ( can it be done without a for loop?)

Since the question is morphing along, I'll add a second answer to the current question. You cannot tell MATLAB to automatically...

4 days ago | 0

| accepted

Setting a lower bound on the values of an array ( can it be done without a for loop?)

I'm sorry, but you are simply wrong. When a number is TOO small, it does not turn into a NaN. It turns into either zero, or -inf...

4 days ago | 0

Unable to find explicit solution

Why do you think a solution exists at all? And, if a any solution does exist, then IF any solution exists at all, there will alm...

5 days ago | 0

Why does my nonlinear (exponential or trigonometric) model fit poorly?

BASINS OF ATTRACTION: Since I will be unable to make my first answer too long, let me expand on the issue of poor starting valu...

5 days ago | 0

fit function does not work correctly

I wrote a long question (and answer) just yesterday, that goes into depth on the why and how to solve this. https://www.mathwor...

5 days ago | 0

| accepted

Why does my nonlinear (exponential or trigonometric) model fit poorly?

Suppose I wish to fit some data to a simple exponential model. I'll make up some data, then add moderate noise to the data. X ...

5 days ago | 0

| accepted

Question

Why does my nonlinear (exponential or trigonometric) model fit poorly?

This is a question I see often online by users of curve fitting tools. Typically, I see someone try to use a tool like the curve...

6 days ago | 3 answers | 2

### 3

answersi have a problem with my plot, i am not getting any values and the system gives me an empty plot.

Look at what you did. a = 0;5; First, maybe you think this creates a vector in a? Instead, it just sets a to 0. Then it dumps...

6 days ago | 1

| accepted

How do I use multi-parameters in my optimization problem for fminunc?

No. It won't look like that. fminunc cannot somehow magically know how you intend the inputs to be used. Computers cannot read y...

6 days ago | 0

How to find a limit without syms and limit function

You can use my limest function. It is on the file exchange. >> fun= @(x) (x.^3 - 1)./(x-1) fun = function_handle with value...

10 days ago | 0

not getting left eigen vector

If you want the set of left and right eigenvectors, we see in the help for eig... [V,D,W] = eig(A) also produces a full mat...

11 days ago | 0

Optimization of a series of linear equations to maximize profit

They are not differential equations. So I'm not sure what you were possibly thinking. They are LINEAR equations. But you do no...

12 days ago | 0

Approximate dots area by circle and define its radius

You cannot do anything until you decide what you need to do. You say to need to approximate it with a circle. If the circle wer...

12 days ago | 0

plotting number bigger than double realmax

If your numbers are greater than realmax, then you can't. Period. You cannot represent a number greater than realmax. You cannot...

14 days ago | 0

How do I calculate a logarithm of a variable base?

Just write your own function. Save it on your search path. Now you have it. [log(10),logb(10)] % natural log [log10(5),logb(5,...

14 days ago | 1

How to read a very large number, say of 700 digits?

You don't understand. A 700 digit number is not representable in double precision. Even if it were, then roughly 684 of those di...

14 days ago | 1

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How to obtain the 3 × 3 symmetric covariance matrix of a 3x1 matrix?

If all you have are three numbers, then a covariance matrix is meaningless. There are two cases we might consider, since I have ...

15 days ago | 0

How to plot three variables in matlab?? A*cos(w*t+theta)

It certainly is possible. All you need is a multi-dimensional holographic monitor to view it. Sadly, these are hard to come by, ...

15 days ago | 2

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Stop Bit growth for power computation

Case 1: You CANNOT raise a double precision number to a power such that it exceeds flintmax (2^53 - 1) and expect the result to ...

16 days ago | 1