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visits member for 2 years, 5 months
seen Jul 7 at 1:10

Consultant (environmental stats a specialty) and teacher.


Apr
30
comment Generating convex polyhedron from face planes?
@level1807 Good catch! Although I haven't tested it, it looks like a bug to me--evidently from a typographical error. The intention was to to compare the chopped values to zero, not to chop the results of a floating point comparison (which makes little sense anyway). I will go ahead and modify the code in this answer. Thanks again.
Apr
30
comment Generating convex polyhedron from face planes?
@level1807 I cannot say: as you can see from my example, I had no trouble with $50$ faces. The possible explanations for what you are encountering could range from differences among MMA versions through floating-point errors through some kind of bug in my code. It could be related to the limitations described in the introduction to this answer. If you want to resolve this you will need to find as simple as possible an example that can be reproduced and then offer it in a new question. Good luck!
Feb
6
comment Plot a 2D vector path onto a surface
@rm-rf Thank you for the tip and the kind words. I am over-extended moderating two other SE sites. Although both have elected two more moderators in the interim, the work is still too much. I hope to return here at some point.
Jan
16
comment How to check if a 3D point is in a planar polygon?
@Rainer Assuming the points are very close to coplanar, the smallest eigenvalue will be close to zero and much smaller than the other two. The eigenvectors associated with the other eigenvalues--that is, the first two principal components--thereby span a plane passing close to most of the points. This gives two solutions: (1) The cross product of the first two principal components will be a normal vector for that plane. (2) The third principal component will be orthogonal to the other two, whence it too is a normal vector for the plane.
Nov
7
comment NMinimize usage
@Frederik You are right: I made that typo early on and because everything worked, I did not catch it. Your suggestion $x = 1+y^2$ accomplishes what was intended and everything goes through as planned. Thank you for reading this post so carefully and well!
Oct
24
comment Insert $+$, $-$, $\times$, $/$, $($, $)$ into $123456789$ to make it equal to $100$
@xzczd Thank you! That is a useful insight and looks to be a likely explanation for the failure.
May
24
comment AstronomicalData and Planetary Heliocentric (x,y,z) Velocity Components
+1 Consider a central difference instead: it should be more accurate. It makes a substantial difference, by the way: it affects the third sig. fig. in your example, because it reflects the net planetary acceleration during the course of 12 hours.
May
24
comment How to calculate the volume of a convex hull?
That doesn't quite seem to be a duplicate, @andre, because there the data are given in a different form. In principle there should be a formula for computing the volume in terms of a suitable integrals over the curve.
May
23
comment Finding integral points on a surface
The equation is a cubic (parameterized by $n$), so it's possible a more efficient method to obtain all solutions can be found by computing generators of its group of integral points. But typically these groups are so small--I haven't found one larger than $12$ yet--as a practical matter it might not be worth the effort.
May
23
comment Finding integral points on a surface
You're welcome. You shouldn't be so quick to accept this answer, though: this community does a great job of improving answers over the course of a day or two. Much better answers might appear soon :-).
May
22
comment CorrelationTest small bug?
It seems to me they are testing the same hypotheses but are using different approximations to the sampling distribution of the test statistic.
May
22
comment Composition: how to make a day and night world map?
This question also has an answer (using MMA illustrations) on the GIS site.
May
21
comment How can I detect an ellipse in a photo?
+1 That's a real tour de force, beautifully described. Impressive.
May
21
comment Unexpected behavior of confidence bands for data presenting two regions with uneven noise
Heteroscedasticity can be complicated: no longer does it suffice to stipulate that all residuals have the same variance, so now you need a model not only for the data values, but also an equally complicated one for the variances of their residuals. You're in the realm of highly specialized coding. If you know what you're doing, MMA is a great platform for writing that code; otherwise, look to specialized platforms like R and hope someone has contributed a model that might be appropriate for your situation.
May
21
comment Speeding up a numerical constrained quadratic optimization
@OleksandrR's suggestion to use a quadratic programming solution typically requires 0.02 seconds or less when applied to problems of the same size having random coefficients.
May
21
comment How can I detect an ellipse in a photo?
Substantial progress towards a solution can be found by reviewing these related posts. A Hough transform looks attractive.
May
20
comment Problem with the Plot of Functions and PlotLegends
What specifically is wrong with the output?
May
20
comment Solar System Orbital Parameters
Just in case you are interested in accuracy, it might be worth noting that the system of ODEs described in your previous question at mathematica.stackexchange.com/questions/25039/… is not what is usually meant by an "n-body simulation," because it does not account for interactions among the bodies: it's just a collection of independent central field solutions. It might be a fine approximation for times close to the initial time but will be observably incorrect.
May
20
comment Find all the integer numbers $a$, $b$, $c$, $d$, $e$, $f$, $k$ to this equation have three integer different solutions?
The question, which evidently is missing several words, now reads as if you are asking for all integral values of $a,\ldots,k$ in the interval $[-10,10]$ for which your equation has three distinct integral solutions. Is this a correct interpretation?
May
18
comment Using single replacement rule to convert algebraic expression
You might find Alternatives to be useful.