47,131 reputation
169188
bio website uoregon.edu/~noeckel
location
age
visits member for 2 years, 9 months
seen 7 mins ago

Sep
25
comment Derivatives of Convolutions with Integration by Parts and Delta Functions
Can you define p[x] with some additional lines of code?
Sep
24
comment How to replace the first occurance in an expression instead of replacing all?
Yes, that works. But now you'd have to do the same if it were Times instead of Plus... anyway, you put your finger exactly on the reason why the original didn't work, and changing the attributes is certainly a way to make it work.
Sep
24
comment How to replace the first occurance in an expression instead of replacing all?
It won't work with this example: HoldForm[7 + x + 2 + 4 + x] /. x + a___ -> 4 + a because the order isn't preserved.
Sep
23
comment Elementwise operation, part deux
@DanielLichtblau Ah, so that's why Nike started their "Just Do it" insurgency in 1988! It all makes sense now...
Sep
23
comment Dirac Gamma matrix Algebra in different dimension
Related: Dirac Matrices in Higher Dimensions (Wolfram Demo)
Sep
22
comment Export textanalysis to pdf
This works on Macs, too. Annoyingly, the ImportString trick doesn't work with "PDF" in this example, so "EPS" does indeed seem to be unavoidable.
Sep
22
comment Creating a foggy image
I think the exact analogue to what you did in Photoshop is actually an example in the documentation for SetAlphaChannel. You can get a more 3D effect by using Raster3D, but that would probably be overkill.
Sep
21
comment Intersection of two vector spaces
To give a useful answer, it would help to know what have you tried and what you mean by "manually" computing matrices (i.e., does a sequence of Mathematica commands count as "manual"?).
Sep
20
comment How to “Copy as Unicode” from a Notebook?
On my Mac this answer works fine (including the copy button) in Mathematica versions 8 and 10.
Sep
19
comment Export[] performance
On my Mac, with Mathematica version 8, the h5 export is still faster than the BinaryWrite: the result were: {0.575183,test.h5}, and {0.588023,Null} respectively.
Sep
19
comment Problems with plotting the imaginary part of an eigenvalue
Yes, I would answer the same way. Maybe you can add that k=0 is a point of degeneracy, which causes the kink in the plot: the eigenvalue is not required to follow a differentiable curve through such a point. However, you could define the "correct" plot by switching between eigenvectors 3 and 4 at k=0.
Sep
18
comment Solution to a T+U = E equation
I've added some more explanation and also addressed your original InverseFunction approach in my edited answer.
Sep
17
comment HoldForm doesn't hold form
@Mr.Wizard I'll look at it when I get a chance to upgrade. I'm currently happily using a computer with only version 8 installed.
Sep
16
comment Strange opacity behavior
Thanks for doing the comparison of approaches in your answer here, which I strangely overlooked until now.
Sep
15
comment Strange behaviour of MMA in derivatives of some standard functions
I tried to figure out what the question is about, but I honestly can't think of any way to answer here.
Sep
15
comment Strange opacity behavior
I think you should do the second code block first. Lately I've been using that trick in the form of a function, see e.g. here.
Sep
13
comment Mathematica isn't sure whether a sum of two positives is positive or not
@jim Since I don't know what prevents Refine from working, it will be very hard to give a general cure for this problem. Here is another idea: see if you get an empty set by applying Assuming[0 < x < 1 && 0 < y < 1 && 0 < z < 1, FindInstance[$Assumptions && Not[statement], {x, y, z}]]. That's all I can think of right now.
Sep
12
comment Plotting an orbit using first principles
If the goal is to do the time-stepping yourself, then I think it's a duplicate of RK4 Gravity Simulator.
Sep
12
comment What is the equation to plot a vertical line?
A related question is Calling Correct Function for Plotting DiracDelta. Not the same because the plot has value 0 away from the spike.
Sep
12
comment Solve Laplace equation using NDSolve
@SantiCarmesí For the gradient and field strength, you can use my answer here. The error message in the potential plot appears because the inner boundary isn't sampled quite accurately. One can get rid of this by first defining rm=RegionMember[\[CapitalOmega]] and then replacing the plot argument sol[x,y] by If[rm[{x,y}],sol[x,y],100].