12.9.2 Greatest hits from previous weeks

12.9.2.1 What does "Tag Integer is Protected" message mean?

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=6738

(Newbie question) 
 
For every function I define I get this message: 
When I close and reopen Mathematica, and re-enter the function definition, I don't get the message anymore. What does it mean? 
[warning-messages]
 

asked by stevenvh https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=1450 Score of 16

answered by J. M.'s missing motivation https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=50 Score of 17

12.9.2.2 Plotting a Phase Portrait

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=14160

I'm trying to plot a phase portrait for the differential equation 
$$x'' - (1 - x^2) x' + x = 0.5 \cos(1.1 t)\,.$$ 
The primes are derivatives with respect to $t$. I've reduced this second order ODE to ... 
[plotting] [differential-equations]
 

asked by covertbob https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=4528 Score of 48

answered by David Slater https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=1746 Score of 42

12.9.2.3 Is there something like DensityPlot3D to visualize atomic orbitals?

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=32378

I'm visualizing some hydrogen like atomic orbitals.  For looking at plane slices of the probability density, the  DensityPlot  function works well, and with something ... 
[plotting] [physics] [chemistry]
 

asked by Peeter Joot https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=10 Score of 47

answered by xslittlegrass https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=1364 Score of 29

12.9.2.4 Is it possible to export the equations from Mathematica to MATLAB?

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=14035

Is it possible to export the output expressions from Mathematica computations (e.g., equations) in valid MATLAB syntax? 
[export] [matlab]
 

asked by Seyhmus Güngören https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=4256 Score of 47

answered by rm -rf https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=5 Score of 51

12.9.2.5 What does # mean in Mathematica?

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=19035

I asked Mathematica to compute the following 
 
 Solve[c (1-x)^2-x^(1/4) == 0, x] 
 
 
and it returned this: 
x = Root[#1^8 c^4 - 8 #1^7 c^4 + 28 #1^6 c^4 - 56 #1^5 c^4 +... 
[syntax] [core-language]
 

asked by ronanymous https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=5730 Score of 53

answered by Dr. belisarius https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=193 Score of 82

12.9.2.6 Is there a syntax for single-line comments for notebooks?

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=1496

I'm aware that I can use  (* ... *)  to comment out stuff in a notebook. Many languages have a syntax for single-line comments, such as 
 
... 
[syntax] [comment]
 

asked by Peeter Joot https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=10 Score of 87

answered by Szabolcs https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=12 Score of 90

12.9.2.7 How to tell Mathematica that certain variables are real/imaginary, integer-valued, etc

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=66273

I'm trying to expedite some quantum mechanical calculations (expectation values etc.) by running them through Mathematica. When I say, for example, 
... 
[functions] [complex] [assumptions] [variable-definitions]
 

asked by Matthew Brunetti https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=22304 Score of 58

answered by SquareOne https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=19960 Score of 42