Definitions
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Eqn

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.

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 eqn language is similar to the mathematical component of TeX, which appeared several years later, but is simpler and less complete.

An independent compatible implementation of the eqn preprocessor has been developed by GNU as part of groff, the GNU version of troff.

Syntax examples

Here is how some of the examples from would be written in eqn (with equivalents in TeX for comparison):

TeX eqn formula
\$a^2\$ a sup 2 $a^2$
\$sum_{k=1}^N k^2\$ sum from { k = 1 } to N { k sup 2 } $sum_\left\{k=1\right\}^N k^2$