3.22.2 Greatest hits from previous weeks

3.22.2.1 Can Mathematica do symbolic linear algebra?

http://mathematica.stackexchange.com/questions/3242/can-mathematica-do-symbolic-linear-algebra

 
For instance, is there some way I can say "let A and B be arbitrary real $m\times n$ and $k\times m$ matrices, Simplify[Transpose[Transpose[A].Transpose[B]]]" and Mathematica would simplify it to B.A? 
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- asked by nikie (35 votes), answered by helen (18 votes)

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

http://mathematica.stackexchange.com/questions/66273/how-to-tell-mathematica-that-certain-variables-are-real-imaginary-integer-value

 
I'm trying to expedite some quantum mechanical calculations by running them through Mathematica. Expectation values and stuff like that. When I say, for example, 
 
u[x_] := Sqrt[2/L] * Sin[Pi * n * x /  ...
                                                                                  

                                                                                  
 

- asked by Matthew Brunetti (9 votes), answered by SquareOne (7 votes)