Special Characters

Built into Mathematica are a large number of special characters intended for use in mathematical and other notation. "Listing of Named Characters" 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 Esc inf Esc. You can set up additional aliases using the InputAliases notebook option discussed in "Options for Expression Input and Output".

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 Esc \infty Esc. Mathematica also supports aliases such as Esc ∞ Esc 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.

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 , while is equivalent to the function Union.

Among ordinary characters such as E and , 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 .

Characters such as and are treated by Mathematica as letters—just like ordinary keyboard letters like or .

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.

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.

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.

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

Traditional mathematical notation occasionally uses ordinary letters as operators. An example is the in a differential such as 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 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 Esc dd Esc.

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.

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 , and is named \[TildeTilde] rather than, say, .

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.

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.

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, Esc ^ Esc is the alias for \[Wedge].

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, Esc -> Esc is the alias for or \[Rule], while Esc && Esc is the alias for or \[And].

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

Notice that the alias for \[VerticalBar] is Esc | Esc, while the alias for the somewhat more common \[VerticalSeparator] is Esc | Esc. 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.

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.