11.4.2 Greatest hits from previous weeks

11.4.2.1 What does "Tag Integer is Protected" message mean?
11.4.2.2 How do I plot a plane EM wave?
11.4.2.3 Why should I avoid the For loop in Mathematica?
11.4.2.4 Are you interested in purchasing David Wagner's "Power programming with Mathematica"?
11.4.2.5 Is there an open source implementation of Mathematica-the-language?
11.4.2.6 Finding Limits in several variables
11.4.2.7 How to plot an ellipse?
11.4.2.1 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
11.4.2.2 How do I plot a plane EM wave?

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=1987

I would like to display an electromagnetic (EM) wave.  I have written code that works, but it does not "shade" the area between the graph and the axes.  Both the ... 
[graphics] [physics] [visualization]
 
asked by Rainforest Frog https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=202 Score of 24
answered by J. M.'s missing motivation https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=50 Score of 18
11.4.2.3 Why should I avoid the For loop in Mathematica?

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=134609

Some people advise against the use of  For  loops in Mathematica. Why?  Should I heed this advice?  What is wrong with  For ?  What ... 
[core-language] [coding-style] [faq]
 
asked by Szabolcs https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=12 Score of 118
answered by Szabolcs https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=12 Score of 125
11.4.2.4 Are you interested in purchasing David Wagner's "Power programming with Mathematica"?

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=16485

I recently contacted McGraw-Hill to see if they have a mechanism in place for printing out-of-print books that are still of interest.  Specifically, I asked about "Power programming with Mathematica" ... 
[reference-request]
 
asked by Todd Allen https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=249 Score of 165
11.4.2.5 Is there an open source implementation of Mathematica-the-language?

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=4454

I've seen questions before such as "What is the best open-source equivalent for Mathematica?", but that specific question (and that line of inquiry in general) cares more about the computer algebra ... 
[programming] [open-source]
 
asked by sblom https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=178 Score of 131
answered by Simon https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=34 Score of 95
11.4.2.6 Finding Limits in several variables

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=21544

Is there a way to find a limit of a multivariable function, like 
 
$$\lim_{(x,y)\to (0,0)} f(x,y)$$ 
with Mathematica? 
 
When $f$ is continuous, we can use 
$$\lim_{(x,y)\to (0,0)} f(x,y)=\lim_{(x,0)\... 
[functions] [calculus-and-analysis]
 
asked by Dominic Michaelis https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=5899 Score of 34
answered by whuber https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=91 Score of 44
11.4.2.7 How to plot an ellipse?

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=20100

I'm new to Mathematica, and I'm finding it difficult to plot an ellipse. 
I tried using 
 
 Plot[(x/5)^2 + (y/3)^2 == 1, {x, -5, 5}, {y, -3, 3}] 
 
 
but I'm getting ... 
[plotting]
 
asked by Sriram https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=6075 Score of 20
answered by Jens https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=245 Score of 33