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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 build-in help files how these characters are to be used.

Can anyone demonstrate?

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Formal symbols are 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. –  rm -rf Feb 5 '13 at 20:50
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up vote 12 down vote accepted

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:

Mathematica graphics

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

Mathematica graphics

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This is a nice issue for the beginner's pitfalls question too –  belisarius Feb 6 '13 at 2:41
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