7.18.3 Can you answer these questions?

7.18.3.1 What is "Flatten" doing to my elements?

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=246395

Here's some of my code. x1 results in returning the position of a list in a {{{n1}},{{n2}}...}. Im trying to flatten it such that the list reads out as {n1,n2,n3....} some how it's adding elements. 
... 
[list-manipulation]
 

asked by MT Cup https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=79502 1 vote

7.18.3.2 Inconsistent boundary and initial conditions when solving sine Gordon equation

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=246596

I have been trying to solve the following sine Gordon equation 
$\partial_{x,x} u(x,t) - \partial_{t,t} u(x,t) - \sin (u(x,t)) - \alpha \partial_t u(x,t) + \gamma = 0$, 
for $x \in [0,15]$ and $t \in [0,... 
[differential-equations]
                                                                                  

                                                                                  
 

asked by t Lê Thành https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=68852 1 vote

7.18.3.3 Step-by-step solution for a Limit

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=246560

Is it possible in Mathematica to get a step-by-step evaluation of some functions? Such that Mathematica is outputting not only the result but all the stages that have led to it? If so, how does one do ... 
[calculus-and-analysis]
 

asked by putti.123 https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=80178 1 vote