Abstrakt
We investigate logics and classes of problems below Fagin's existential second-order logic (ESO) and above Feder and Vardi's logic for constraint satisfaction problems (CSP), the so called monotone monadic SNP without inequality (MMSNP). It is known that MMSNP has a dichotomy between P and NP-complete but that the removal of any of these three restrictions imposed on SNP yields a logic that is Ptime equivalent to ESO: so by Ladner's theorem we have three stronger sibling logics that are nondichotomic above MMSNP.
In this paper, we explore the area between these four logics, mostly by considering guarded extensions of MMSNP, with the ultimate goal being to obtain logics above MMSNP that exhibit such a dichotomy.