I developed an Android application and I want to make a survey/questionaire about the usability, user interface, user experience etc. of my application to get some feedback from users. But I don't know which kind of questions to ask...

I would like to know If there are some links/tools/books that can help me.


Question: Recursion depth of 1024 exceeded during evaluation
Answer: So much wrong there...

a = 2;
tx = 2^(3/2);
x[t_] := a Cos[2 Pi t/tx] - Cos[2 Pi t];
y[t_] := a Sin[2 Pi t/tx] - Sin[2 Pi t];
f[t_] := x[t] + I y[t];

f[20]

(* -1 + 2 Cos[10 Sqrt[2] π] + 2 I Sin[10 Sqrt[2] π] *)


Don't use uppercase initials for your symbols - you might clash with built-ins.

Don't use some external user-defined global symbol inside a definition in the manner you have unless there's a good reason to do so (I left that as is, but it's bad practice).

Read the documentation, learn the difference between Set (=) and SetDelayed (:=).

Read What are the most common pitfalls awaiting new users? on this very site. Great resource for new users.

Go through the tutorials in the Mathematica documentation. Very worthwhile.