eqn

eqn is a computer program used for typesetting mathematical equations and formulas. It was developed as part of the Unix operating system and serves as a preprocessor for the troff document formatting system. The program allows users to describe mathematical expressions using a simple markup language, which is then converted into properly formatted equations suitable for publication.

The eqn preprocessor works by interpreting equation descriptions written in a specialized syntax and translating them into troff commands. Users can specify mathematical expressions, fractions, integrals, summations, and various mathematical symbols through intuitive textual representations. The system handles proper positioning, sizing, and spacing of mathematical elements automatically.

eqn was originally created by Brian Kernighan and Lorinda Cherry at Bell Labs in the 1970s. It became an integral part of the Unix system and influenced the development of later mathematical typesetting systems. The program's design philosophy emphasized simplicity and ease of use, allowing technical writers and researchers to focus on mathematical content rather than formatting details.

The syntax of eqn uses keywords and constructs that resemble natural language descriptions of mathematical expressions. For example, fractions are created using the "over" keyword, subscripts with "sub", and superscripts with "sup". This approach makes mathematical markup more readable and writable compared to purely symbolic approaches.

eqn has been widely influential in technical computing and documentation systems. Many modern equation editors and mathematical typesetting systems trace their conceptual origins to eqn's approach. While newer systems like LaTeX have largely superseded eqn in contemporary usage, it remains an important historical milestone in the development of computer-based mathematical typography and continues to be available in Unix-like systems for legacy document processing.