I noticed that Mathematica has a set of special 'Formal' characters such as \[FormalA], \[FormalB] , etc.. In the front end, it looks like a character with a dot above and below:
It is not clear from the built-in documentation how these characters are to be used.
Would someone demonstrate?
Very often, especially new Mathematica users stumble over the following error: They gave, maybe hours ago, a symbol a value like x=3, and later they try to use it where a function really expects a symbol:
This leads of course to an error, because the Minimize call does not see the x, but only its value. The same happens when you try to derive this with D or Dt or Integrate expressions. Although the error message is very clear, most people get very confused and try to change their whole calculation.
Exactly for those situations, the formal parameter characters are made. They all share the attribute Protected which states that you cannot simply assign a value to them. Therefore, you can use them in situations where you maybe derive expressions or proof formulas or where you minimize an expression
I would like to point out that the shortcut to write out formal symbols is :$a:, where : is typed using Esc. See the documentation here.
Protected, so they can't be assigned values (by default). So you can use it as a way of ensuring that your definitions don't have stray values. It has been used several times on this site (see some of the posts here for examples), although I don't think a question has been asked explicitly about it. $\endgroup$\[FormalDelta]is but\[FormalPhi]is notProtected. According to TechSupport, this is by design, as some formal symbols are used internally, some are not. This is aggrevating, as one cannot rely on that a formal symbol has no global value set by the user. $\endgroup$\[Formal]characters were introduced? $\endgroup$