eqn (software)

Part of the troff suite of Unix document layout tools, eqn is a preprocessor that formats equations for printing. A similar program, neqn, accepted the same input as eqn, but produced output tuned to look better in nroff. The eqn program was created in 1974 by Brian Kernighan and Lorinda Cherry. It was implemented using yacc compiler-compiler.^[1]

The input language used by eqn allows the user to write mathematical expressions in much the same way as they would be spoken aloud. The language is defined by a

Eqn was written using the yacc parser generator.^[1]

Syntax examples

Here is how some examples would be written in eqn^[3] (with equivalents in TeX for comparison):

TeX	eqn	formula
a^2	a sup 2	${\displaystyle a^{2}}$
\sum_{k = 1}^N k^2	sum from { k = 1 } to N { k sup 2 }	${\displaystyle \sum _{k=1}^{N}k^{2}}$
x = {-b \pm \sqrt{b^2 - 4ac} \over 2a}	x = {-b +- sqrt{b sup 2 - 4ac}} over 2a	${\displaystyle x={-b\pm {\sqrt {b^{2}-4ac}} \over 2a}}$

Spaces are important in eqn; tokens are delimited only by whitespace characters, tildes ~, braces {} and double-quotes "". Thus f(pi r sup 2) results in ${\textstyle f(pir^{2)}}$, whereas f( pi r sup 2 ) is needed to give the intended ${\textstyle f(\pi r^{2})}$.

References

External links

• Typesetting Mathematics, User's Guide (Second Edition)
• UNIX man page for neqn(1) at the Wayback Machine (archived August 8, 2007)
• eqn(1) – Plan 9 Programmer's Manual, Volume 1

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