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3.10.1 Special Characters

Built into Mathematica are a large number of special characters intended for use in mathematical and other notation. Section A.12.1 gives a complete listing.
Each special character is assigned a full name such as \[Infinity]. More common special characters are also assigned aliases, such as AliasIndicatorinfAliasIndicator, where  stands for the  key. You can set up additional aliases using the InputAliases notebook option discussed in Section 2.10.11.
For special characters that are supported in standard dialects of TeX, Mathematica also allows you to use aliases based on TeX names. Thus, for example, you can enter \[Infinity] using the alias AliasIndicator\inftyAliasIndicator. Mathematica also supports aliases such as AliasIndicator&infinAliasIndicator based on names used in SGML and HTML.
Standard system software on many computer systems also supports special key combinations for entering certain special characters. On a Macintosh, for example, Option-5 will produce  in most fonts. With the notebook front end Mathematica automatically allows you to use special key combinations when these are available, and with a text-based interface you can get Mathematica to accept such key combinations if you set an appropriate value for $CharacterEncoding.

Ways to enter special characters.

In a Mathematica notebook, you can use special characters just like you use standard keyboard characters. You can include special characters both in ordinary text and in input that you intend to give to Mathematica.
Some special characters are set up to have an immediate meaning to Mathematica. Thus, for example,  is taken to be the symbol Pi. Similarly,  is taken to be the operator GreaterEqual, while  is equivalent to the function Union.

 and  have immediate meanings in Mathematica.

 or \[Union] is immediately interpreted as the Union function.

 or \[SquareUnion] has no immediate meaning to Mathematica.

Among ordinary characters such as E and i, some have an immediate meaning to Mathematica, but most do not. And the same is true of special characters.
Thus, for example, while  and  have an immediate meaning to Mathematica,  and  do not.
This allows you to set up your own definitions for  and  .

 has no immediate meaning in Mathematica.

This defines a meaning for  .

Now Mathematica evaluates  just as it would any other function.

Characters such as  and  are treated by Mathematica as letters--just like ordinary keyboard letters like a or b.
But characters such as  and  are treated by Mathematica as operators. And although these particular characters are not assigned any built-in meaning by Mathematica, they are nevertheless required to follow a definite syntax.

 is an infix operator.

The definition assigns a meaning to the  operator.

Now  can be evaluated by Mathematica.

The details of how input you give to Mathematica is interpreted depends on whether you are using StandardForm or TraditionalForm, and on what additional information you supply in InterpretationBox and similar constructs.
But unless you explicitly override its built-in rules by giving your own definitions for MakeExpression, Mathematica will always assign the same basic syntactic properties to any particular special character.
These properties not only affect the interpretation of the special characters in Mathematica input, but also determine the structure of expressions built with these special characters. They also affect various aspects of formatting; operators, for example, have extra space left around them, while letters do not.

Types of special characters.

In using special characters, it is important to make sure that you have the correct character for a particular purpose. There are quite a few examples of characters that look similar, yet are in fact quite different.
A common issue is operators whose forms are derived from letters. An example is  or \[Sum], which looks very similar to  or \[CapitalSigma].
As is typical, however, the operator form  is slightly less elaborate and more stylized than the letter form  . In addition,  is an extensible character which grows depending on the summand, while  has a size determined only by the current font.

Different characters that look similar.

In cases such as \[CapitalAlpha] versus A, both characters are letters. However, Mathematica treats these characters as different, and in some fonts, for example, they may look quite different.

The result contains four distinct characters.

Traditional mathematical notation occasionally uses ordinary letters as operators. An example is the d in a differential such as dx that appears in an integral.
To make Mathematica have a precise and consistent syntax, it is necessary at least in StandardForm to distinguish between an ordinary d and the  used as a differential operator.
The way Mathematica does this is to use a special character  or \[DifferentialD] as the differential operator. This special character can be entered using the alias AliasIndicatorddAliasIndicator.

Mathematica uses a special character for the differential operator, so there is no conflict with an ordinary d.

When letters and letter-like forms appear in Mathematica input, they are typically treated as names of symbols. But when operators appear, functions must be constructed that correspond to these operators. In almost all cases, what Mathematica does is to create a function whose name is the full name of the special character that appears as the operator.

Mathematica constructs a CirclePlus function to correspond to the operator  , whose full name is \[CirclePlus].

Out[12]//FullForm=

This constructs an And function, which happens to have built-in evaluation rules in Mathematica.

Out[13]//FullForm=

Following the correspondence between operator names and function names, special characters such as  that represent built-in Mathematica functions have names that correspond to those functions. Thus, for example,  is named \[Divide] to correspond to the built-in Mathematica function Divide, and  is named \[Implies] to correspond to the built-in function Implies.
In general, however, special characters in Mathematica are given names that are as generic as possible, so as not to prejudice different uses. Most often, characters are thus named mainly according to their appearance. The character  is therefore named \[CirclePlus], rather than, say \[DirectSum], and  is named \[TildeTilde] rather than, say, \[ApproximatelyEqual].

Different operator characters that look similar.

There are sometimes characters that look similar but which are used to represent different operators. An example is \[Times] and \[Cross]. \[Times] corresponds to the ordinary Times function for multiplication; \[Cross] corresponds to the Cross function for vector cross products. The  for \[Cross] is drawn slightly smaller than  for Times, corresponding to usual careful usage in mathematical typography.

The \[Times] operator represents ordinary multiplication.

The \[Cross] operator represents vector cross products.

The two operators display in a similar way--with \[Times] slightly larger than \[Cross].

In the example of \[And] and \[Wedge], the \[And] operator--which happens to be drawn slightly larger--corresponds to the built-in Mathematica function And, while the \[Wedge] operator has a generic name based on the appearance of the character and has no built-in meaning.

You can mix \[Wedge] and \[And] operators. Each has a definite precedence.

Out[17]//FullForm=

Some of the special characters commonly used as operators in mathematical notation look similar to ordinary keyboard characters. Thus, for example,  or \[Wedge] looks similar to the ^ character on a standard keyboard.
Mathematica interprets a raw ^ as a power. But it interprets  as a generic Wedge function. In cases such as this where there is a special character that looks similar to an ordinary keyboard character, the convention is to use the ordinary keyboard character as the alias for the special character. Thus, for example, AliasIndicator^AliasIndicator is the alias for \[Wedge].

The raw ^ is interpreted as a power, but the AliasIndicator^AliasIndicator is a generic wedge operator.

A related convention is that when a special character is used to represent an operator that can be typed using ordinary keyboard characters, those characters are used in the alias for the special character. Thus, for example, AliasIndicator->AliasIndicator is the alias for  or \[Rule], while AliasIndicator&&AliasIndicator is the alias for  or \[And].

AliasIndicator->AliasIndicator is the alias for \[Rule], and AliasIndicator&&AliasIndicator for \[And].

Out[19]//FullForm=

The most extreme case of characters that look alike but work differently occurs with vertical bars.

Different types of vertical bars.

Notice that the alias for \[VerticalBar] is AliasIndicator |AliasIndicator, while the alias for the somewhat more common \[VerticalSeparator] is AliasIndicator|AliasIndicator. Mathematica often gives similar-looking characters similar aliases; it is a general convention that the aliases for the less commonly used characters are distinguished by having spaces at the beginning.

Conventions for special character aliases.

The notebook front end for Mathematica often allows you to set up your own aliases for special characters. If you want to, you can overwrite the built-in aliases. But the convention is to use aliases that begin with a dot or comma.
Note that whatever aliases you may use to enter special characters, the full names of the characters will always be used when the characters are stored in files.