Pajek

Pajek is a Windows-based software package for the analysis and visualization of large networks (graphs). It is designed to enable researchers to perform complex network analyses on very large data sets, with an emphasis on performance and scalability.

Developed by Vladimir Batagelj and Andrej Mrvar at the University of Ljubljana, Pajek was released in the 1990s. The name Pajek comes from the Slovenian word for spider, reflecting its focus on exploring expansive networks.

Key features include a comprehensive set of network analysis tools (centrality measures, components, clustering, k-core decomposition, shortest paths) and visualization capabilities. It supports multiple graph layouts (e.g., Kamada–Kawai, Fruchterman–Reingold, Sugiyama for hierarchical layouts) and provides scripting and batch processing options for reproducibility. Pajek can handle graphs with thousands to millions of elements depending on memory, and produces vector or bitmap outputs for publication.

Files and formats: Pajek uses its own native .net format for networks and offers converters to and from other formats; it can import and export several standard graph formats to facilitate interoperability with other tools.

Usage and impact: Pajek has been widely used in social network analysis, bibliometrics, and transport and biological networks, especially in academic settings. It remains a common teaching and research tool despite the emergence of newer software.