Inclusion Logic and Fixed Point Logic

Abstract

We investigate the properties of Inclusion Logic, that is, First Order Logic with Team Semantics extended with inclusion dependencies. We prove that Inclusion Logic is equivalent to Greatest Fixed Point Logic, and we prove that all union-closed first-order definable properties of relations are definable in it. We also provide an Ehrenfeucht-Fra\"iss\'e game for Inclusion Logic, and give an example illustrating its use.