# 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**

4x4 matrix with numbers from 1 to 4 in specific order

Hint: What does flip do? Could you use it to "flip" the order of those numbers?

ungefär 10 timmar ago | 0

I want to find the value of k

syms x1 x2 x3 x=[x1;x2;x3]; f1=x(1)^3+12*x(2)+1; f2=x(2)^9+5*x(1)-3; f3=x(2)*x(3)^2+8; Here is the matrix k. k=[diff(f1,x(...

ungefär 14 timmar ago | 2

smooth step function with rise time

You cannot solve the problem you claim to want to solve using a simple logistic. It does not have the flexiibility to control bo...

ungefär 16 timmar ago | 0

Automated Bisection Method on a function tan(a)-tan(b)=0

I'm sorry, but it is provably impossible to make bisection (or any scheme like it) always find all solutions of an arbitrarily c...

ungefär 17 timmar ago | 0

Why do I see a singular matrix warning here?

The issue is not a problem with eig, except in our expectations of what eig can do. And those expecations are fueled by our expe...

en dag ago | 0

Question

Why do I see a singular matrix warning here?

I've posted this question as a "response" to a question posed by someone else, but as an answer to another question. Since I can...

en dag ago | 1 answer | 1

### 1

answerFind the file number when a condition meets

Beginners do this sort of thing. They are tentative, almost afraid to use MATLAB, even when they think they need to. So they lea...

3 dagar ago | 0

1D randomwalk matlab tutorial on youtube

I think you need to read things like the syntax of various tools, like while. You also need to learn the syntax of just how to w...

3 dagar ago | 0

Generate all possible combinations summing up to a given number

These are commonly known to mathematicians as integer partitions, thus the ways we can write an integer as a sum of smaller inte...

4 dagar ago | 0

How many terms of series are required to get accuracy upto three decimal point. That is series sum correct upto 3.141

@ajeet sahu - what you do not seem to understand is that the script you have shown IS performing the correct iterations, The pro...

4 dagar ago | 0

| accepted

Constants not recognized in integral

You are still thinking that you are using some other language, where you need to declare variables as doubles. This line does n...

7 dagar ago | 0

How can i write this equation: d = obs + sqrt(alfa*Cd^0.5)*randn(nd, ne)

Do I understand you correctly? obs is an array of size 51x71x3? Or is it a vector of that length? Regardless... alfa is a scala...

7 dagar ago | 0

Array indices must be positive integers or logical values.

It does not matter that you WANT V(0) to be something. MATLAB does not support zero based indexing. PERIOD. However, nothing st...

8 dagar ago | 1

plotting a for loop returns Empty Graph

You are not plotting ANYTHING in a for loop. You may think you are. Where are the plot statements? AFTER THE LOOP TERMINATES. Af...

8 dagar ago | 0

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...

9 dagar ago | 0

| accepted

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...

10 dagar ago | 2

| accepted

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 ...

10 dagar 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...

10 dagar 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...

11 dagar 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...

11 dagar 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...

11 dagar 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...

11 dagar 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...

11 dagar 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...

11 dagar ago | 0

| accepted

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...

11 dagar 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...

12 dagar 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...

12 dagar 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...

12 dagar 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...

13 dagar 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...

13 dagar ago | 0

| accepted