- Semidefinite functions on categories
- The Electronic Journal of Combinatorics
- Volume | Issue number
- 16 | 2
- Number of pages
- Document type
- Faculty of Science (FNWI)
- Korteweg-de Vries Institute for Mathematics (KdVI)
- Freedman, Lovász and Schrijver characterized graph parameters that can be represented as the (weighted) number of homomorphisms into a fixed graph. Several extensions of this result have been proved. We use the framework of categories to prove a general theorem of this kind. Similarly as previous results, the characterization uses certain infinite matrices, called connection matrices, which are required to be positive semidefinite.
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