Special Characters: Mathematical and Other Notation
Built into the Wolfram Language 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 EscinfEsc. 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, the Wolfram Language also allows you to use aliases based on TeX names. Thus, for example, you can enter \[Infinity] using the alias Esc\inftyEsc. The Wolfram Language 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 the Wolfram System automatically allows you to use special key combinations when these are available, and with a text‐based interface you can get the Wolfram System to accept such key combinations if you set an appropriate value for $CharacterEncoding.
|■ Use a full name such as \[Infinity]|
|■ Use an alias such as EscinfEsc|
|■ Use a TeX alias such as Esc\inftyEsc|
|■ Use an SGML or HTML alias such as Esc∞Esc|
|■ Click a button in a palette|
|■ Use a special key combination supported by your computer system|
In a Wolfram System 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 the Wolfram System.
Some special characters are set up to have an immediate meaning to the Wolfram System. 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 i, some have an immediate meaning to the Wolfram System, but most do not. And the same is true of special characters.
But characters such as and are treated by the Wolfram System as operators. And although these particular characters are not assigned any built‐in meaning by the Wolfram System, they are nevertheless required to follow a definite syntax.
The details of how input you give to the Wolfram System 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, the Wolfram System 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 Wolfram System 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.
As is typical, however, the operator form is slightly less elaborate and more stylized than the letter form . In addition, is an extensible character that grows depending on the summand, while has a size determined only by the current font.
In cases such as \[CapitalAlpha] versus A, both characters are letters. However, the Wolfram System treats these characters as different, and in some fonts, for example, they may look quite different.
To make the Wolfram System 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 the Wolfram System does this is to use a special character or \[DifferentialD] as the differential operator. This special character can be entered using the alias EscddEsc.
When letters and letter‐like forms appear in Wolfram System 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 the Wolfram System 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 Wolfram System functions have names that correspond to those functions. Thus, for example, is named \[Divide] to correspond to the built‐in Wolfram System function Divide, and is named \[Implies] to correspond to the built‐in function Implies.
In general, however, special characters in the Wolfram Language 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].
There are sometimes characters that look similar but that 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 Wolfram System 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.
The Wolfram System 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].
Notice that the alias for \[VerticalBar] is Esc␣|Esc, while the alias for the somewhat more common \[VerticalSeparator] is Esc|Esc. The Wolfram Language 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.
|EscnnnEsc||built‐in alias for a common character|
|Esc␣nnnEsc||built‐in alias for similar but less common character|
|Esc.nnnEsc||alias globally defined in a Wolfram System session|
|Esc,nnnEsc||alias defined in a specific notebook|
The notebook front end for the Wolfram System 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.