# 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?

12 hours 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(...

16 hours 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...

18 hours 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...

19 hours 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...

1 day 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...

1 day 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 days 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 days 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 days 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 days ago | 0

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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 days 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...

8 days 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 days 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 days 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...

10 days 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...

10 days 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 days 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 days 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 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...

11 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...

11 days ago | 0

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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 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...

11 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...

11 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...

11 days ago | 1

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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 days ago | 0

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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 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...

13 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...

13 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...

13 days ago | 0

| accepted