Graph quasivarieties
          Lehtonen, Erkko; Pöschel, Reinhard  
          
          Acta Sci. Math. (Szeged), 86 (2020), 31–50  
          http://doi.org/10.14232/actasm-019-528-9  
           
          Introduced by C. R. Shallon in 1979, graph algebras establish a useful connection between graph theory and universal algebra. This makes it possible to investigate graph varieties and graph quasivarieties, i.e., classes of graphs described by identities or quasi-identities. In this paper, graph quasivarieties are characterized as classes of graphs closed under directed unions of isomorphic copies of finite strong pointed subproducts.  
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