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              Abstract
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 We show that the 2-Segal spaces (also called decomposition spaces) of Dyckerhoff and
 Kapranov, and Gálvez-Carrillo, Kock, and Tonks have a natural analogue within
 simplicial sets, which we call quasi-2-Segal sets, and that the two ideas enjoy a
 similar relationship as the one Segal spaces have with quasicategories. In particular,
 we construct a model structure on the category of simplicial sets whose fibrant
 objects are the quasi-2-Segal sets which is Quillen equivalent to a model structure for
 complete 2-Segal spaces (where our notion of completeness comes from one of the
 equivalent characterizations of completeness for Segal spaces). We also prove a path
 space criterion, which says that a simplicial set is a quasi-2-Segal set if and only if its
 path spaces (also called décalage) are quasicategories, as well as an edgewise
 subdivision criterion.
  
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              Keywords
              
                2-Segal, quasi-2-Segal, complete 2-Segal, decomposition
                space, simplicial set, model category, quasicategories,
                Segal spaces, Cisinski model structure, path space
                criterion
               
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              Mathematical Subject Classification
              
                Primary: 18N50, 55U35
               
              
                Secondary: 18N40, 18N60, 55U10
               
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              Milestones
              
                Received: 15 April 2022
               
              
                Revised: 26 October 2022
               
              
                Accepted: 26 November 2022
               
              
                Published: 4 June 2023
               
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