# 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**

A compact way to find single digit numbers (i.e. numbers between 0 and 9) and replace them with two digit numbers

In MATLAB, you CANNOT store a number in a numeric format as one that has a leading zero. If you want to convert the numbers to ...

1 día ago | 0

| accepted

How to calculate the rotation of this egg?

Find a bounding box, where the box can be rotated, not a box that is aligned with the axes. Find the orientation (angle of incl...

1 día ago | 0

| accepted

How can I find negative factorial ? (-0.5)!

Simple. Yes, you could use MATLAB, IF you knew that the extension of the function factorial(n) onto the real line is just gamma(...

1 día ago | 1

Out of memory error

You have 20000 variables. So fmincon will be computing a Hessian matrix internally of size 20000*20000 = 4e8. But that matrix is...

1 día ago | 2

Why does smoothdata give bad results at the beginning and end of a dataset?

Think about it. This is really classic behavior, completely expected. Imagine the smoothing routine as a mathematical implementa...

1 día ago | 2

Value of infinite integral is different

First, What is the correct value of this integral? syms k K = (k.^3).*(1+(2.35.*(k).^(2/3))) Just looking at it, I see a func...

2 días ago | 1

How can I get the symbolic steady state vector of a Markov Chain?

Easy, peasy. For example, given a simple Markov process, described by the 3x3 transition matrix T. T = [.5 .2 .3;.1 .4 .5;.1 .1...

3 días ago | 1

How to extrapolate given data to find more number of points?

Your words ask how to do interpolation. However, then you ask about extrapolation. The two problems are VERY different. To quo...

4 días ago | 0

Array indices must be positive integers or logical values?

MATLAB does not allow an index of 0. The FIRST element of an array or vector is indexed as x(1). In your loop, what was the ran...

4 días ago | 0

MAXIMUM MATRIKS IN MATLAB

Since you did not say which one to leave non-zero, I'll asssume that all values equal to the maximum are retained. A = randi(20...

4 días ago | 0

| accepted

Backslash does not provided the solution with the smallest 2-norm

It seems the gist of your question comes down to the idea that for a wide matrix, MATLAB should (in your eyes only) always produ...

5 días ago | 0

Solving Trigonometric Equations with More Than One Variables

You need to understand that first, the use of solve like this will probably fail. You have 4 equations, but only 3 unknowns. Sol...

6 días ago | 0

| accepted

I need an expression between two symbols R and a connected within a large order determinant.

Sorry, but not going to happen. Ever. Why not? There are multiple reasons why this will fail, so you are tilting at impossibly h...

7 días ago | 0

Function, dot, help me for solution

Your problem has NOTHING to do with dots. This is a common mistake we see made here. Suppose you have a pair of function handle...

8 días ago | 1

Equation to Matlab code

(I really hope I am not doing your homework here, but the question is not posed as if it is, even though I assume you are a stud...

8 días ago | 0

| accepted

How to get elapsed time of a code in matlab ?

Sorry, but no. A prediction of the time required for some arbitrary piece of code will never happen. It cannot happen. Well, no...

9 días ago | 2

Optimization of points generating inside a sphere

The common scheme that is advised is to generate points randomly, then if a point is too close to a neighbor, then you reject it...

10 días ago | 0

why covariance matrix Should be positive and symmetric in mahalanobis distance

SIGH. Multiplying a covariance matrix by its transpose is NOT what you want to do! If it is already a covariance matrix, that op...

10 días ago | 0

How do we interpolate a data set with noise?

Um, you generally don't want to interpolate noisy data! That is just a good way to amplify the noise. I did show many years ago,...

11 días ago | 0

| accepted

n as the natural numbers

There are many ways to interpret this question. If the goal is simply to TEST to see if n is a natural number, then you might do...

11 días ago | 0

how to use surf function?

Almost always, when someone says they have a single vector of z values and they want to use surf, they mean that they have a lis...

12 días ago | 0

Is there an easier way to index diagonal elements of a matrix?

A = diag(randi(100,10,1)); n = size(A,1); A(sub2ind([n,n],[3 6 7 9],[3 6 7 9])) = 1000; A If you understand how matrix ele...

14 días ago | 1

| accepted

Symbolic solve with user-specified precision

Your belief has preciously little value. Faith in mathematics tends to be a waste of mental energy. Let me expand your equations...

14 días ago | 0

I created an array of size 255 of pseudo-random integers . I want to access the value at a particular index of the array how can I do it ?

Basically, you need to look at the getting started tutorials. Try the MATLAB Onramp. r0 = uint8(randi(255,255,1)); r0(158)

14 días ago | 0

| accepted

Is it possible to (non-linear) minimize x^y by choosing both x and y?

Can MATLAB solve it? Yes. Your doubt is only you not knowing how to solve the problem. In fact, simplest is to transform the pr...

15 días ago | 0

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isprime function seems to have poor performance

As a follow-up, I've also compared the MATLAB isprime to that of Python and Mathematica. On significanty large numbers, The MATL...

16 días ago | 2

Where to get a zip code for free 30 days trial

This is not a question about MATLAB. However, the answer is easy. CONTACT sales/customer service. Just contact the MathWorks dir...

16 días ago | 1

Is it possible to generate an encrypted function that can be used a limited number of times?

Is it possible? It depends on how good or technically naive are your users. How determined would they be? You can "encrypt" a f...

17 días ago | 1

Optimization - find a max on a complicated function

You maximize a function using a tool that minimizes it, by finding the minimum of -f(x). How to do it? Use fminbnd. Set the bou...

18 días ago | 1

finding the orthogonal vectors for a series of vectors

I think you are confused. Really, this is not a difficult problem. You need to understand the linear algebra of it, or you will ...

19 días ago | 3