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Squares $K$-theory and 2-Segal spaces

Maxine E. Calle and Maru Sarazola

Vol. 11 (2026), No. 2, 261–308
Abstract

We define an S-construction for squares categories and introduce a class of squares categories we call proto-Waldhausen which capture the properties required for the S-construction to model the K-theory space. The primary question we investigate is when the S-construction of a squares category produces a 2-Segal space. We show that the answer to this question is affirmative when the squares category satisfies certain “stability” conditions.

Keywords
2-Segal spaces, squares $K$-theory, Waldhausen $S_\bullet$-construction, double categories
Mathematical Subject Classification
Primary: 19D99
Secondary: 18G90, 18N10, 55U10
Milestones
Received: 9 October 2024
Revised: 7 January 2026
Accepted: 6 March 2026
Published: 9 April 2026
Authors
Maxine E. Calle
Department of Mathematics
University of Pennsylvania
Philadelphia, PA
United States
Maru Sarazola
Department of Mathematics
University of Minnesota
Minneapolis, MN
United States