6.8.4 Eqn Language Mode

Eqn is another popular formatter for math formulas. It is designed for use with the TROFF text formatter, and comes standard with many versions of Unix. The d E (calc-eqn-language) command selects eqn notation.

The eqn language’s main idiosyncrasy is that whitespace plays a significant part in the parsing of the language. For example, ‘sqrt x+1 + y’ treats ‘x+1’ as the argument of the sqrt operator. Eqn also understands more conventional grouping using curly braces: ‘sqrt{x+1} + y’. Braces are required only when the argument contains spaces.

In Calc’s eqn mode, however, curly braces are required to delimit arguments of operators like sqrt. The first of the above examples would treat only the ‘x’ as the argument of sqrt, and in fact ‘sin x+1’ would be interpreted as ‘sin * x + 1’, because sin is not a special operator in the eqn language. If you always surround the argument with curly braces, Calc will never misunderstand.

Calc also understands parentheses as grouping characters. Another peculiarity of eqn’s syntax makes it advisable to separate words with spaces from any surrounding characters that aren’t curly braces, so Calc writes ‘sin ( x + y )’ in eqn mode. (The spaces around sin are important to make eqn recognize that sin should be typeset in a roman font, and the spaces around x and y are a good idea just in case the eqn document has defined special meanings for these names, too.)

Powers and subscripts are written with the sub and sup operators, respectively. Note that the caret symbol ‘^’ is treated the same as a space in eqn mode, as is the ‘~’ symbol (these are used to introduce spaces of various widths into the typeset output of eqn).

As in LaTeX mode, Calc’s formatter omits parentheses around the arguments of functions like ln and sin if they are “simple-looking”; in this case Calc surrounds the argument with braces, separated by a ‘~’ from the function name: ‘sin~{x}’.

Font change codes (like ‘roman x’) and positioning codes (like ‘~’ and ‘down n x’) are ignored by the eqn reader. Also ignored are the words left, right, mark, and lineup. Quotation marks in eqn mode input are treated the same as curly braces: ‘sqrt "1+x"’ is equivalent to ‘sqrt {1+x}’; this is only an approximation to the true meaning of quotes in eqn, but it is good enough for most uses.

Accent codes (‘x dot’) are handled by treating them as function calls (‘dot(x)’) internally. See TeX and LaTeX Language Modes, for a table of these accent functions. The prime accent is treated specially if it occurs on a variable or function name: ‘f prime prime ( x prime )’ is stored internally as ‘f''(x')’. For example, taking the derivative of ‘f(2 x)’ with a d x will produce ‘2 f'(2 x)’, which eqn mode will display as ‘2 f prime ( 2 x )’.

Assignments are written with the ‘<-’ (left-arrow) symbol, and evalto operators are written with ‘->’ or ‘evalto ... ->’ (see TeX and LaTeX Language Modes, for a discussion of this). The regular Calc symbols ‘:=’ and ‘=>’ are also recognized for these operators during reading.

Vectors in eqn mode use regular Calc square brackets, but matrices are formatted as ‘matrix { ccol { a above b } ... }’. The words lcol and rcol are recognized as synonyms for ccol during input, and are generated instead of ccol if the matrix justification mode so specifies.