Quantified Equality Constraints
Manuel Bodirsky, Hubie Chen · SIAM Journal on Computing · 2010
An equality template is a relational structure with infinite universe whose relations can be defined by Boolean combinations of equalities. We prove a complexity classification for quantified constraint satisfaction problems (QCSPs) over equality templates: These problems are in L (decidable in logarithmic space), NP-hard, or coNP-hard. To establish our classification theorem we combine methods from universal algebra with concepts from model theory.