John D'Errico

Retired

Last seen: 1 day ago | Active since 2006

Followers: 36 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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OH NO! isprime is missing? (Just kidding)
What if you had no isprime utility to rely on in MATLAB? How would you identify a number as prime? An easy answer might be some...

2 months ago | 0

Discussion

Approximating a floating point number with a finite representation in decimal form
I saw an interesting problem on a reddit math forum today. The question was to find a number (x) as close as possible to r=3.6, ...

3 months ago | 0

Discussion

Primes and rough numbers, use of MOD as a large scale test to exclude composite numbers
The GCD approach to identify rough numbers is a terribly useful one, well worth remembering. But at some point, I expect someo...

10 months ago | 2

Discussion

Primes and rough numbers, using GCD as a test for roughness to exclude composite numbers
Yes, some readers might now argue that I used roughness in a crazy way in my last post, in my approach to finding a large twin...

10 months ago | 3

Discussion

Primes and rough numbers, used constructively to search for large primes
How can we use roughness in an effective context to identify large primes? I can quickly think of quite a few examples where w...

10 months ago | 4

Discussion

Primes and Rough Numbers, Basic ideas
What is a rough number? What can they be used for? Today I'll take you down a journey into the land of prime numbers (in MATLA...

10 months ago | 7

Discussion

Answers not displaying symbolic results? FIXED
syms u v atan2alt(v,u) function Z = atan2alt(V,U) % extension of atan2(V,U) into the complex plane Z = -1i*log((U+1i*V)....

1 year ago | 2

Discussion

Using graphs to learn about Kaprekar constant(s)
D.R. Kaprekar was a self taught recreational mathematician, perhaps known mostly for some numbers that bear his name. Today, I...

1 year ago | 8

Discussion

Using MATLAB to find a generative equation for a sequence
This stems purely from some play on my part. Suppose I asked you to work with the sequence formed as 2*n*F_n + 1, where F_n is t...

1 year ago | 8

Discussion

Should the AI apply to all questions posted on Answers?
I saw this post on Answers. https://www.mathworks.com/matlabcentral/answers/2046770-explaining-a-matlab-code/?s_tid=ans_lp_feed...

2 years ago | 3

Discussion

linprog failed to work. why? Did I do something wrong?
I've now seen linear programming questions pop up on Answers recently, with some common failure modes for linprog that people se...

3 years ago | 1

Discussion

Counting the passengers on a train: solving a basic problem using MATLAB
I saw this problem online recently. <https://www.youtube.com/watch?v=-rbozrj9poo Passenger distribution on a train> Not a...

3 years ago | 8